USMLE Microbiology Exam Practice Test
Questions and Answers
- 1.
A 5-year-old child who recently moved from Eastern Europe is brought to the emergency room with difficulty breathing and a low-grade fever. Physical examination reveals a grayish pharyngeal exudate, enlarged cervical lymph nodes and partial soft palate paralysis. Bacteria isolated from the pharyngeal exudate demonstrate exotoxin production in the laboratory. This exotoxin acts via the following mechanism:
- A.
Intracellular protein ribosylation
- B.
Degradation of lecithin
- C.
Increased sensitivity to histamine
- D.
Inactivation of the 60s ribosomal subunit
- E.
Autoimmune activation by antigenic mimicry
Correct Answer
A. Intracellular protein ribosylationExplanation
The correct answer is "Intracellular protein ribosylation". In this scenario, the child's symptoms, including difficulty breathing, fever, grayish pharyngeal exudate, enlarged cervical lymph nodes, and partial soft palate paralysis, suggest a diagnosis of diphtheria. Diphtheria is caused by the bacteria Corynebacterium diphtheriae, which produces an exotoxin called diphtheria toxin. This toxin acts by ribosylating (adding a ribose sugar) to a specific protein within host cells, inhibiting protein synthesis and causing cell death. This mechanism of intracellular protein ribosylation is responsible for the characteristic symptoms and complications of diphtheria.Rate this question:
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- 2.
An 8-year-old Caucasian boy is brought to your office with throat pain, fever and malaise. Physical examination reveals white exudates on his tonsils and swollen anterior cervical lymph nodes. If the boy returns in a month with fatigue, joint pain and chest pain and later in life he develops a heart murmur, which of the following would be the most likely explanation for his condition?
- A.
Protein A-mediated opsonization block
- B.
Bacterial and human epitope homology
- C.
Immune complex deposition
- D.
Exotoxin-induced T-cell receptor activation
- E.
Coronary artery aneurysm formation
Correct Answer
B. Bacterial and human epitope homologyExplanation
The most likely explanation for the boy's condition is bacterial and human epitope homology. This means that the bacteria causing his initial symptoms have antigens that are similar to antigens found in his own body. This can lead to an autoimmune response, where the immune system mistakenly attacks the boy's own tissues, resulting in symptoms such as fatigue, joint pain, chest pain, and eventually a heart murmur. This condition is often seen in certain bacterial infections, such as streptococcal infections, which can lead to rheumatic fever.Rate this question:
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- 3.
A 67-year-old male is hospitalized with low-grade fevers, fatigue and a diastolic murmur at the left sternal border. Blood cultures reveal Gram positive cocci that are catalase-negative and able to grow in 6.5% saline. This patient's medical history is most likely to reveal which of the following procedures in the past month?
- A.
Dental extraction
- B.
Skin biopsy
- C.
Sinus drainage
- D.
Nasal polypectomy
- E.
Cystoscopy
Correct Answer
E. CystoscopyExplanation
The correct answer is cystoscopy. This is because the patient presents with low-grade fevers, fatigue, and a diastolic murmur, which are indicative of infective endocarditis. The presence of Gram positive cocci that are catalase-negative and able to grow in 6.5% saline suggests the causative organism is Streptococcus viridans, which is commonly associated with dental procedures. However, in this case, the patient's medical history is most likely to reveal a cystoscopy, as this procedure carries a risk of introducing bacteria into the bloodstream and causing infective endocarditis.Rate this question:
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- 4.
Infective endocarditis is known to be caused by many different bacterial species. Which of the following scenarios is most consistent with infective endocarditis caused by Streptococcus bovis?
- A.
A 34-year-old female with known mitral stenosis develops low-grade fevers and negative blood cultures
- B.
A 45-year-old male complains of fatigue and exertional dyspnea three weeks after tooth extraction
- C.
A 62-year-old female has a persistent fever after being diagnosed with colon cancer
- D.
A 64-year-old male with repeatedly negative blood cultures has small mitral vegetation on transesophageal echocardiogram
- E.
A 29-year-old male with a persistent fever has tricuspid vegetations and tricuspid regurgitation on transthoracic echocardiogram
Correct Answer
C. A 62-year-old female has a persistent fever after being diagnosed with colon cancerExplanation
This scenario is most consistent with infective endocarditis caused by Streptococcus bovis because it is well known that this bacterial species is commonly associated with colorectal cancer. The persistent fever in this patient after being diagnosed with colon cancer suggests that the infection may have spread from the colon to the heart, causing infective endocarditis.Rate this question:
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- 5.
Gram-positive cocci isolated from the blood of a patient with bacteremia synthesize dextrans from glucose. The bacteria most likely contribute to which of the following pathological states?
- A.
Glomerulonephritis
- B.
Sarcoidosis
- C.
Erythema nodosum
- D.
Migratory polyarthritis
- E.
Anterior uveitis
- F.
Dental caries
Correct Answer
F. Dental cariesExplanation
Gram-positive cocci that synthesize dextrans from glucose are most likely contributing to dental caries. Dextrans are sticky substances that can form a biofilm on the surface of teeth, providing a favorable environment for the growth of bacteria that cause tooth decay. This biofilm, known as dental plaque, can lead to the formation of cavities and other dental problems. Therefore, the presence of these bacteria and their ability to synthesize dextrans suggest a role in the development of dental caries.Rate this question:
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- 6.
Transient bacteremia is demonstrated in a healthy 22-year-old male after a dental procedure. The Gram-positive bacteria isolated from the blood synthesize dextrans from sucrose. Which of the following would be the most likely adherence site for these bacteria?
- A.
Endothelial surface glycoproteins
- B.
Subendothelial collagen
- C.
Subendothelial glycosaminoglycans
- D.
Circulating heparins
- E.
Fibrin-platelet aggregates
Correct Answer
E. Fibrin-platelet aggregatesExplanation
Fibrin-platelet aggregates would be the most likely adherence site for the bacteria in this case. Transient bacteremia refers to the temporary presence of bacteria in the bloodstream, which can occur after certain procedures like dental work. The fact that the bacteria isolated from the blood synthesize dextrans from sucrose suggests that they have the ability to form biofilms. Fibrin-platelet aggregates are commonly found in areas of vascular injury or inflammation, making them a favorable site for bacterial adherence and biofilm formation.Rate this question:
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- 7.
A group of physicians in a developing country is advocating the early empiric use of penicillin in the treatment of clinically diagnosed bacterial pharyngitis, a condition that is quite common in children of that nation. What would you expect to decrease as a result of their policy in the long term?
- A.
Expensive antibiotic requirement
- B.
Nephritic disease
- C.
Deaths caused by diarrhea
- D.
Bronchial asthma hospitalizations
- E.
Cardiac surgeries
- F.
Vaccination needs
Correct Answer
E. Cardiac surgeriesExplanation
The early empiric use of penicillin in the treatment of bacterial pharyngitis can help prevent the development of rheumatic heart disease, a condition that can lead to the need for cardiac surgeries. By treating bacterial pharyngitis promptly with penicillin, the physicians can reduce the incidence of rheumatic heart disease, thus decreasing the need for cardiac surgeries in the long term.Rate this question:
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- 8.
A 62-year-old Caucasian male who recently underwent a mitral valve replacement is having low-grade fevers. He also complains of dyspnea and malaise. Repeated blood cultures grow Gram-positive cocci in clusters that are catalase-positive and coagulase-negative. Which of the following is the best initial treatment for this patient?
- A.
Penicillin G
- B.
Nafcillin
- C.
Vancomycin
- D.
Ciprofloxacin
- E.
Erythromycin
- F.
Ceftriaxone
Correct Answer
C. VancomycinExplanation
The patient in this scenario is presenting with low-grade fevers, dyspnea, and malaise after undergoing a mitral valve replacement. The repeated blood cultures have grown Gram-positive cocci in clusters that are catalase-positive and coagulase-negative. These characteristics are consistent with a staphylococcal infection, specifically coagulase-negative staphylococci. Vancomycin is the best initial treatment choice for this patient because it has excellent activity against Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus (MRSA), which is a concern in patients who have undergone surgery.Rate this question:
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- 9.
A 52-year-old male is having low-grade fevers after recent replacement of his aortic valve. Repeated blood cultures grow gram-positive cocci. Which of the following characteristics is most consistent with Staphylococcus epidermidis as the cause of this patient's symptoms?
- A.
Catalase negative
- B.
Coagulase negative
- C.
Mannitol fermenting
- D.
Novobiocin resistance
- E.
Yellow pigment production
Correct Answer
B. Coagulase negativeExplanation
Staphylococcus epidermidis is a coagulase-negative bacterium, meaning it does not produce the enzyme coagulase. This characteristic is consistent with the patient's symptoms of low-grade fevers after aortic valve replacement, as S. epidermidis is a common cause of postoperative infections, particularly in prosthetic devices. Coagulase-negative bacteria are often associated with healthcare-associated infections and are known for their ability to form biofilms on medical devices.Rate this question:
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- 10.
An 8-year-old male who recently moved to the United States from Asia is brought to the emergency room with throat pain and difficulty breathing. Soon after being admitted to the hospital, he dies of severe heart failure. In the laboratory, bacterial isolates from this patient's tonsils are found to cause rapid death of 8 out of 1 O exposed guinea pigs. Two of the 10 experimental animals survive. The surviving animals most likely possess which of the following antibodies?
- A.
LgA against adhesion proteins
- B.
Opsonizing lgM
- C.
Complement-fixing lgM
- D.
Immobilizing lgG
- E.
LgG against circulating proteins
Correct Answer
E. LgG against circulating proteinsExplanation
The correct answer is "lgG against circulating proteins." This is because the patient's death was caused by severe heart failure, which suggests that the bacteria in his tonsils were able to enter his bloodstream and cause systemic infection. The fact that the bacterial isolates caused rapid death in guinea pigs also indicates that they were able to circulate throughout the body and cause widespread damage. lgG antibodies are the most abundant type of antibody in the bloodstream and are responsible for neutralizing and eliminating circulating pathogens, making them the most likely antibodies present in the surviving animals.Rate this question:
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- 11.
A 29-year-old female presents to clinic with malaise and an extremely painful rash in her genital region. She has never had such symptoms before. Tzanck smear of the genital lesion is positive for multinucleated giant cells. A drug that can cause rapid resolution of this patient's symptoms would most likely:
- A.
Prevent viral attachment to target cells
- B.
Inhibit viral protease activity
- C.
Incorporate into newly replicated viral DNA
- D.
Prevent virion release from infected cells
- E.
Up-regulate interferon synthesis in infected cells
Correct Answer
C. Incorporate into newly replicated viral DNAExplanation
The correct answer is "Incorporate into newly replicated viral DNA." This is because the presence of multinucleated giant cells on the Tzanck smear suggests a viral infection, such as herpes simplex virus. Drugs that incorporate into newly replicated viral DNA, such as nucleoside analogues like acyclovir, can inhibit viral DNA synthesis and prevent further replication of the virus. This leads to rapid resolution of symptoms.Rate this question:
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- 12.
A 17-year-old female presents to your office with dark urine and facial puffiness. She had a minor skin infection several weeks ago that resolved spontaneously. Microscopic examination of the urine sediment shows red blood cell casts. The organism responsible for this patient's symptoms would most likely demonstrate:
- A.
Catalase positivity
- B.
Growth in 6.5% NaCl
- C.
Bile solubility
- D.
Bacitracin sensitivity
- E.
Optochin sensitivity
Correct Answer
D. Bacitracin sensitivityExplanation
The organism responsible for this patient's symptoms would most likely demonstrate bacitracin sensitivity. This information suggests that the organism is likely to be Streptococcus pyogenes, also known as group A Streptococcus. This bacterium is sensitive to bacitracin, which means it will be inhibited by the antibiotic. The presence of dark urine and facial puffiness, along with red blood cell casts in the urine sediment, suggests a possible diagnosis of glomerulonephritis, which can be caused by a Streptococcus pyogenes infection.Rate this question:
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- 13.
A 23-year-old Caucasian female presents to clinic with lower abdominal pain and scant bloody vaginal discharge. She has been sexually active with multiple partners and uses condoms on occasion. She has been treated for "genital infections" in the past but denies any history of pregnancy. Her blood pressure is 11 2/70 mm Hg while supine and 96/60 mm Hg while standing. A pregnancy test is positive. Which of the following microorganisms is most likely responsible for this patient's current condition?
- A.
Gardnerella vaginalis
- B.
Neisseria gonorrhoeae
- C.
Trichomonas vaginalis
- D.
Treponema pallidum
- E.
Herpes simplex virus
- F.
Molluscum contagiosum
Correct Answer
B. Neisseria gonorrhoeaeExplanation
The correct answer is Neisseria gonorrhoeae. This patient's presentation of lower abdominal pain, scant bloody vaginal discharge, history of multiple sexual partners, and positive pregnancy test is consistent with pelvic inflammatory disease (PID), which is most commonly caused by Neisseria gonorrhoeae. Other symptoms such as a history of "genital infections" and previous treatment further support this diagnosis.Rate this question:
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- 14.
A group of investigators is trying to develop an anti-gonococcal vaccine. Their attempts to use pilus components to induce long-lasting immunity would most likely fail because of:
- A.
Antigenic mimicry
- B.
Low molecular weight
- C.
Non-protein structure
- D.
Antigenic variation
- E.
Local immunosuppression
Correct Answer
D. Antigenic variationExplanation
The reason why attempts to use pilus components to induce long-lasting immunity would most likely fail is due to antigenic variation. Gonococci, the bacteria responsible for gonorrhea, are known to have high levels of genetic variability, leading to frequent changes in their surface antigens. This antigenic variation allows the bacteria to evade the immune system and makes it difficult for the immune system to recognize and target them effectively. Therefore, using pilus components, which are part of the bacterial surface, as a vaccine target would not be effective in inducing long-lasting immunity.Rate this question:
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- 15.
A 26-year-old male presents to your office with dysuria and urethral discharge two weeks after having unprotected sexual intercourse with a new partner. Gram stain of the discharge reveals numerous neutrophils with intracellular diplococci. A sample of the discharge is placed on an antibiotic-containing medium, and bacterial colonies are cultured. This medium is best described as:
- A.
Enrichment
- B.
Differential
- C.
Synthetic
- D.
Selective
- E.
Reducing
Correct Answer
D. SelectiveExplanation
The medium used in this scenario is described as selective. This is because it contains antibiotics, which inhibit the growth of certain bacteria while allowing the growth of others. By using this medium, the goal is to selectively promote the growth of the bacteria causing the infection, while inhibiting the growth of other bacteria that may be present in the sample. This helps in isolating and identifying the specific bacteria responsible for the patient's symptoms.Rate this question:
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- 16.
A 7-year-old Caucasian boy is brought to your office with blisters on his face. Some of the blisters have broken and are covered with golden yellow crusts. Exudate microscopy reveals Gram-positive cocci in chains. Which of the following would be a component of the clinical syndrome that may follow such an infection?
- A.
Joint pain and eye redness
- B.
Fatigue and heart murmurs
- C.
Face puffiness and dark urine
- D.
Bilateral facial nerve palsy
- E.
Abdominal pain and jaundice
Correct Answer
C. Face puffiness and dark urineExplanation
The presence of blisters on the boy's face, along with the golden yellow crusts and the identification of Gram-positive cocci in chains in the exudate microscopy, suggests that the boy has impetigo, which is a bacterial skin infection commonly caused by Staphylococcus aureus or Streptococcus pyogenes. In severe cases, such as with a systemic infection, complications can occur. One such complication is post-streptococcal glomerulonephritis, which can present with symptoms like face puffiness and dark urine.Rate this question:
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- 17.
A 34-year-old woman comes to the physician because of itching around her vulva and vaginal discharge. She has no other complaints. Her other medical problems include chronic allergic rhinitis, for which she takes intranasal corticosteroids. She had an episode of acute bacterial sinusitis 2 week ago and received the appropriate therapy. Pelvic examination shows an erythematous vulva and a thick, adherent, "cottage cheeselike" vaginal discharge. The rest of the physical examination is within normal limits. Microscopic examination of the vaginal discharge reveals budding cells. Which of the following is the most likely underlying cause of her current condition?
- A.
Decreased thickness of the vaginal epithelium
- B.
Decreased glycogen concentration in the vaginal epithelium
- C.
Decreased number of Gram-positive bacteria in the vagina
- D.
Increased alkaline secretions by the cervical mucosa
- E.
Depressed T-lymphocyte response to mucosal stimuli
Correct Answer
C. Decreased number of Gram-positive bacteria in the vaginaExplanation
The presence of a thick, adherent "cottage cheeselike" vaginal discharge, along with microscopic examination revealing budding cells, suggests a yeast infection. Yeast infections are commonly caused by an overgrowth of Candida species, which are a type of fungus. The normal vaginal flora includes a balance of bacteria, including Gram-positive bacteria, which help maintain a healthy vaginal environment and prevent overgrowth of yeast. Therefore, a decreased number of Gram-positive bacteria in the vagina can disrupt this balance and contribute to the development of a yeast infection.Rate this question:
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- 18.
A 34-year-old woman comes to the physician complaining of fever, dysuria, and flank pain. Her temperature is 37.8 C (100 F), blood pressure is 122/78 mm Hg, and pulse is 84/min. Physical examination elicits suprapubic and costovertebral angle tenderness. Urine dipstick analysis reveals positive leukocyte esterase and nitrite. A subsequent urine culture grows colonies of motile Gram-negative rods demonstrating a green metallic sheen on eosin methylene blue (EMB} agar and hemolysis on blood agar. Which of the following is the most important bacterial factor responsible for this patient's condition?
- A.
Capsule
- B.
Heat-labile enterotoxin
- C.
O antigen
- D.
Fimbrial antigen
- E.
Lipid A
Correct Answer
D. Fimbrial antigenExplanation
The most important bacterial factor responsible for this patient's condition is the fimbrial antigen. Fimbriae are hair-like structures found on the surface of bacteria, and they play a crucial role in the attachment of bacteria to host cells. In this case, the presence of fimbrial antigen suggests that the bacteria causing the infection have the ability to adhere to the urinary tract epithelium, leading to symptoms such as dysuria and suprapubic tenderness. Additionally, the positive leukocyte esterase and nitrite on urine dipstick analysis indicate the presence of a urinary tract infection, further supporting the role of the fimbrial antigen in this patient's condition.Rate this question:
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- 19.
A 23-year-old Caucasian female is brought to the ER with fever, vomiting, diarrhea and muscle pain. Her blood pressure is 90/50 mm Hg and pulse is 120/min. Physical examination reveals erythroderma, and pelvic exam reveals a tampon in the vagina. The activation of which of the following cells is primarily responsible for this patient's condition?
- A.
Mast cells and eosinophils
- B.
Basophils and macrophages
- C.
Neutrophils and B lymphocytes
- D.
Macrophages and T lymphocytes
- E.
Platelets and mast cells
Correct Answer
D. Macrophages and T lymphocytesExplanation
The patient presents with symptoms of fever, vomiting, diarrhea, muscle pain, low blood pressure, and a tampon in the vagina. These findings suggest toxic shock syndrome (TSS), which is caused by an immune response to toxins produced by Staphylococcus aureus or Streptococcus pyogenes. Macrophages and T lymphocytes play a crucial role in the immune response against these bacteria. Macrophages phagocytose the bacteria and present antigens to T lymphocytes, which then activate and release cytokines, leading to systemic symptoms such as fever and hypotension. Therefore, the activation of macrophages and T lymphocytes is primarily responsible for this patient's condition.Rate this question:
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- 20.
A 72-year-old man is brought to the emergency department from a skilled nursing facility because of altered mental status, skin flushing, and a high fever. His blood pressure is 60/20 mmHg, and heart rate is 11 O/min. If blood cultures are positive for E coli, which of the following is the most likely source of bacteremia in this patient?
- A.
Small intestine
- B.
Colon
- C.
Urinary tract
- D.
Biliary tree
- E.
Lungs
- F.
Oropharynx
- G.
Skin
Correct Answer
C. Urinary tractExplanation
The most likely source of bacteremia in this patient is the urinary tract. The patient's altered mental status, skin flushing, and high fever are consistent with sepsis, and the positive blood cultures for E. coli suggest a urinary tract infection. Urinary tract infections are common in older adults and can lead to bacteremia, especially in patients with underlying comorbidities or residing in skilled nursing facilities. The low blood pressure and low heart rate may be indicative of septic shock, a severe form of sepsis.Rate this question:
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- 21.
A 59-year-old female is brought to the ER with fever, skin flushing, and an altered level of consciousness. Her blood pressure is 50/20 mmHg, and her heart rate is 120/min. If blood cultures are positive for E coli, which of the following bacterial factors is most likely responsible for this patient's current condition?
- A.
Capsule
- B.
Heat-stable exotoxin
- C.
O antigen
- D.
Fimbrial antigen
- E.
Lipid A
- F.
Flagellar antigen
Correct Answer
E. Lipid AExplanation
Lipid A is the lipid component of lipopolysaccharide (LPS), which is found in the outer membrane of Gram-negative bacteria like E. coli. It is a potent endotoxin that can cause a severe immune response in the host. In this case, the patient's symptoms of fever, skin flushing, and altered level of consciousness, along with low blood pressure and high heart rate, are consistent with septic shock. Lipid A is known to trigger the release of inflammatory mediators, leading to the systemic inflammatory response seen in septic shock. Therefore, Lipid A is most likely responsible for this patient's current condition.Rate this question:
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- 22.
A 39 year-old paraplegic man with an indwelling bladder catheter presents to the emergency room complaining of twenty-four hours of rigors, nausea, and vomiting. His temperature is 38.9° C (102.0° F). Urinalysis shows 3+ leukocyte esterase and numerous white blood cells. Urine and blood cultures grow non-lactose fermenting gram-negative bacilli. Which of the following pathogens is the most likely culprit?
- A.
Escherichia coli
- B.
Pseudomonas aeruginosa
- C.
Klebsiella pneumoniae
- D.
Staphylococcus saprophyticus
- E.
Enterococcus cloacae
- F.
Ureaplasma urealyticum
Correct Answer
B. Pseudomonas aeruginosaExplanation
The most likely culprit in this case is Pseudomonas aeruginosa. This is because the patient's symptoms of rigors, nausea, and vomiting, along with a high temperature, are consistent with a urinary tract infection. The presence of leukocyte esterase and numerous white blood cells in the urinalysis suggests an inflammatory response in the urinary tract. Pseudomonas aeruginosa is a common cause of urinary tract infections in patients with indwelling bladder catheters, especially in those with underlying conditions such as paralysis. Additionally, the growth of non-lactose fermenting gram-negative bacilli in both urine and blood cultures further supports Pseudomonas aeruginosa as the likely pathogen.Rate this question:
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- 23.
Genital examination of a 31-year-old female reveals yellow discharge from the cervical os that demonstrates abundant neutrophils on light microscopy. The patient will most likely experience which of the following as a sequela of this disease?
- A.
Endometrial hyperplasia
- B.
Hydatidiform mole
- C.
Ovarian cancer
- D.
Polycystic ovary syndrome
- E.
Infertility
Correct Answer
E. InfertilityExplanation
The presence of yellow discharge from the cervical os along with abundant neutrophils suggests the presence of an infection. Infections of the female reproductive system, such as pelvic inflammatory disease (PID), can lead to scarring and damage to the reproductive organs. This scarring can result in blockage or damage to the fallopian tubes, making it difficult for the egg to travel from the ovary to the uterus, leading to infertility. Therefore, the most likely sequela of this disease is infertility.Rate this question:
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- 24.
A 24-year-old female presents to your office with burning urination, urgency and frequency. She is sexually active. Urine cultures show catalase-positive, gram-positive cocci. The organism responsible for this patient's symptoms is most likely to be:
- A.
Coagulase positive
- B.
Hemolytic
- C.
Novobiocin resistant
- D.
DNase positive
- E.
Yellow pigment producer
Correct Answer
C. Novobiocin resistantExplanation
The correct answer is Novobiocin resistant. Novobiocin is an antibiotic that is used to differentiate between coagulase-negative staphylococci (which are resistant to novobiocin) and Staphylococcus saprophyticus (which is sensitive to novobiocin). In this case, the patient's symptoms of burning urination, urgency, and frequency, along with the presence of catalase-positive, gram-positive cocci in the urine cultures, suggest a urinary tract infection. Staphylococcus saprophyticus is a common cause of urinary tract infections in young sexually active females, and it is typically resistant to novobiocin. Therefore, the most likely organism responsible for this patient's symptoms is novobiocin resistant.Rate this question:
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- 25.
Group A Streptococci demonstrate significant resistance to phagocytic killing when placed in fresh human blood. This resistance can be most effectively overcome by adding antibodies to which of the following?
- A.
Hyaluronate
- B.
Protein M
- C.
Streptolysin O
- D.
DNase
- E.
Streptokinase
- F.
Teichoic acid
Correct Answer
B. Protein MExplanation
Group A Streptococci demonstrate significant resistance to phagocytic killing when placed in fresh human blood. This resistance can be most effectively overcome by adding antibodies to Protein M.Rate this question:
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- 26.
A 45-year-old sailor comes to the physician with groin pain and swelling. He first noticed a sore on his penis but ignored it because it was not painful and disappeared after a few days. Three weeks later, he began to experience painful swelling in his inguinal region with inflammation of the overlying skin and the eventual formation of several draining ulcers. He also complains of mild fever and malaise that began around the same time as his groin symptoms. Cell scrapings from his lesions show cytoplasmic inclusion bodies. Which of the following is the most likely cause of this patient's condition?
- A.
Haemophilus ducreyi
- B.
K!ebsiella inguinale
- C.
Herpes simplex virus
- D.
Treponema pallidum
- E.
Chlamydia trachomatis
Correct Answer
E. Chlamydia trachomatisExplanation
The patient's symptoms of painful swelling in the inguinal region with inflammation of the overlying skin and the eventual formation of several draining ulcers, along with the presence of cytoplasmic inclusion bodies in cell scrapings from his lesions, are consistent with lymphogranuloma venereum (LGV), which is caused by Chlamydia trachomatis. Haemophilus ducreyi causes chancroid, not the symptoms described. Klebsiella inguinale, also known as Donovanosis, presents with painless genital ulcers that do not resolve spontaneously. Herpes simplex virus causes painful vesicles that ulcerate and heal within a few weeks. Treponema pallidum causes syphilis, which presents with a painless chancre that heals spontaneously.Rate this question:
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- 27.
A 22-year-old female presents with severe pain and swelling in her right knee, left elbow, and left wrist. Joint aspiration of the knee reveals an opaque exudate with high neutrophil content and intracellular organisms. This patient's symptoms could have most likely been prevented by:
- A.
Methotrexate therapy
- B.
Proper sore throat treatment
- C.
Timely vaccination
- D.
Barrier contraception
- E.
Avoiding intravenous drug abuse
Correct Answer
D. Barrier contraceptionExplanation
Barrier contraception refers to the use of physical barriers such as condoms or diaphragms to prevent the transmission of sexually transmitted infections (STIs) and unwanted pregnancies. The patient in this scenario presents with symptoms suggestive of a septic arthritis, which is typically caused by an infection. The presence of intracellular organisms in the joint aspiration suggests a potential sexually transmitted infection. Therefore, practicing barrier contraception would have reduced the risk of acquiring an STI, which could have prevented the symptoms of joint infection in this patient.Rate this question:
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- 28.
A 24-year-old female presents to your office with burning during urination, urine clouding and urinary frequency. She denies fever, chills and flank pain. She had a similar episode before that was treated with antibiotics. She has no other medical problems and does not use tobacco, alcohol or drugs. Her vital signs are stable. Physical examination shows suprapubic tenderness. Which of the following bacteria is most likely to be isolated from this patient's urine?
- A.
Klebsiella pneumoniae
- B.
Samonella typhi
- C.
Salmonella enteritidis
- D.
Campylobacter jejuni
- E.
Shigella dysenteriae
- F.
Proteus mirabilis
- G.
Yersinia enterocolitica
- H.
Haemophilus influenzae
- I.
Vibrio cholerae
- J.
Streptococcus pyogenes
- K.
Streptococcus pneumoniae
- L.
Escherichia coli
Correct Answer
L. Escherichia coliExplanation
Escherichia coli is the most likely bacteria to be isolated from this patient's urine because it is the most common cause of urinary tract infections (UTIs), especially in young, sexually active females. The patient's symptoms of burning during urination, urine clouding, and urinary frequency are consistent with a UTI. Additionally, the absence of fever, chills, and flank pain suggests that the infection has not yet spread to the kidneys. E. coli is a gram-negative bacterium that is commonly found in the gastrointestinal tract and can ascend into the urinary tract, causing infection.Rate this question:
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- 29.
A 34-year-old male presents to your office with a painless penile ulcer several weeks after an episode of unprotected sexual intercourse. Which of the following is most useful in diagnosing Treponema pallidum infection in this patient?
- A.
Ulcer exudate microscopy after Gram staining
- B.
Ulcer exudate microscopy with darkfield illumination
- C.
Ulcer exudate cultures on enrichment media
- D.
Blood cultures on differential media
- E.
Antitoxin detection in the serum
Correct Answer
B. Ulcer exudate microscopy with darkfield illuminationExplanation
Ulcer exudate microscopy with darkfield illumination is the most useful in diagnosing Treponema pallidum infection in this patient. Darkfield microscopy allows for the visualization of the spirochetes, such as Treponema pallidum, which causes syphilis. This method is particularly effective in detecting the presence of the bacteria in the ulcer exudate, as it provides high contrast and enhances the visibility of the spirochetes. Other methods, such as Gram staining or blood cultures, may not be as effective in diagnosing syphilis specifically.Rate this question:
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- 30.
A 32-year-old Caucasian male develops profuse watery diarrhea with abdominal cramps, nausea and vomiting during a trip to Mexico. The diarrhea resolved in a couple of days without antibiotic treatment. This patient's symptoms are most likely related to:
- A.
Intestinal invasion
- B.
'Stacked-brick' intestinal adhesion
- C.
Shiga-like toxin
- D.
Cholera-like toxin
- E.
Endotoxin release
Correct Answer
D. Cholera-like toxinExplanation
The patient's symptoms of profuse watery diarrhea, abdominal cramps, nausea, and vomiting are consistent with cholera. Cholera is caused by the bacterium Vibrio cholerae, which produces a toxin called cholera toxin. This toxin causes the intestines to secrete large amounts of water, leading to the characteristic watery diarrhea. The fact that the diarrhea resolved without antibiotic treatment also supports the diagnosis of cholera, as antibiotics are not typically needed for treatment.Rate this question:
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- 31.
A 46-year-old male who just returned from a mission trip to Latin America presents to your office complaining of fever, myalgias, dull abdominal pain, and a week-long history of watery diarrhea that has recently become bloody. Physical examination reveals a fever of 38.9° C (102.0° F), hepatosplenomegaly, and rose-colored spots at the periumbilical area. Which of the following is the most likely cause of this patient's symptoms?
- A.
Escherichia coli
- B.
Klebsiella pneumoniae
- C.
Salmonela typhi
- D.
Salmonella enteritidis
- E.
Campylobacter fetus
- F.
Proteus mirabilis
- G.
Yersinia enterocolitica
- H.
Haemophilus inffuenzae
- I.
Helicobacter pylori
- J.
Vibrio cholerae
- K.
Streptococcus pyogenes
- L.
Streptococcus pneumoniae
Correct Answer
C. Salmonela typhiExplanation
The patient's symptoms, including fever, myalgias, abdominal pain, and bloody diarrhea, along with the presence of hepatosplenomegaly and rose-colored spots at the periumbilical area, are consistent with a diagnosis of typhoid fever. Salmonella typhi is the causative agent of typhoid fever, which is commonly acquired through contaminated food or water in regions with poor sanitation. Other options such as Escherichia coli, Salmonella enteritidis, and Campylobacter fetus can also cause gastroenteritis, but they do not typically present with hepatosplenomegaly and rose-colored spots. Therefore, Salmonella typhi is the most likely cause of this patient's symptoms.Rate this question:
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- 32.
A 24-year-old male presents to your ER with severe nausea and vomiting two hours after consuming a mayonnaise-containing salad at an outdoor picnic. His condition improved significantly in several hours. You inform the patient that his symptoms were caused by which of the following?
- A.
Exotoxin formed prior to ingestion
- B.
Exotoxin formed after ingestion
- C.
Endotoxin
- D.
Bacterial invasion of gut mucosa
- E.
Parasitic infection
Correct Answer
A. Exotoxin formed prior to ingestionExplanation
The patient's symptoms of severe nausea and vomiting occurring two hours after consuming the salad suggest that the cause of his symptoms was an exotoxin formed prior to ingestion. Exotoxins are toxins released by bacteria that can cause symptoms even before the bacteria themselves are present in the body. In this case, the mayonnaise-containing salad likely contained bacteria that produced exotoxins, which caused the patient's symptoms.Rate this question:
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- 33.
Healthy volunteers orally inoculated with pathogenic strains of Clostridium difficile do not develop signs of infection. Which of the following protective mechanisms is most likely responsible?
- A.
Cell-mediated immunity
- B.
Mucosal lgA antibodies
- C.
Gastric acidity
- D.
Intestinal biomass
- E.
Rapid gastrointestinal transit
Correct Answer
D. Intestinal biomassExplanation
Intestinal biomass refers to the total mass of microorganisms present in the intestine. These microorganisms play a crucial role in maintaining the balance of the gut microbiota and preventing the colonization of pathogenic bacteria like Clostridium difficile. Therefore, a high intestinal biomass can help prevent the development of infection by inhibiting the growth and colonization of pathogenic strains. This is why healthy volunteers orally inoculated with pathogenic strains of Clostridium difficile do not develop signs of infection.Rate this question:
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- 34.
A 65-year-old male taking amoxicillin for sinusitis is hospitalized with diarrhea and abdominal cramps. Complete blood count shows leukocytosis. This toxin responsible for his current condition primarily damages which of the following components of intestinal mucosal cells?
- A.
Mitochondrial energy production
- B.
Cytoskeleton integrity
- C.
Apical ion transport
- D.
Ribosomal protein synthesis
- E.
Cell membrane integrity
Correct Answer
B. Cytoskeleton integrityExplanation
The correct answer is cytoskeleton integrity. The patient's symptoms of diarrhea and abdominal cramps suggest damage to the intestinal mucosal cells. The cytoskeleton is responsible for maintaining the structural integrity of cells and plays a crucial role in cell movement and shape. Damage to the cytoskeleton can disrupt the normal functioning of intestinal cells, leading to symptoms such as diarrhea and abdominal cramps.Rate this question:
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- 35.
An outbreak of water-borne gastroenteritis with a high mortality is reported in some Asian countries. It is caused by oxidase-positive, gram-negative rods that grow well on highly alkaline media. Which of the following patients would most likely require the smallest infective dose to initiate the infection?
- A.
Patients previously exposed to the bacteria
- B.
Patients with chronic pancreatitis
- C.
Patients on omeprazole treatment
- D.
Patients with chronic hepatitis
- E.
Patients with peptic ulcer disease
Correct Answer
C. Patients on omeprazole treatmentExplanation
Patients on omeprazole treatment would most likely require the smallest infective dose to initiate the infection. Omeprazole is a proton pump inhibitor that reduces the production of gastric acid in the stomach. Gastric acid plays a crucial role in killing ingested bacteria, so when its production is reduced, the stomach becomes less acidic and more susceptible to bacterial infection. Therefore, patients on omeprazole treatment have a higher risk of developing infections, including water-borne gastroenteritis caused by oxidase-positive, gram-negative rods that grow well on highly alkaline media.Rate this question:
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- 36.
A 22-year-old male ingests a solution containing "C-labeled urea. He is then asked to blow into a tube, and the labeled carbon is detected in his breath samples. This test is most likely part of the evaluation for which of the following conditions?
- A.
Bronchiectasis
- B.
Pulmonary fibrosis
- C.
Congenital heart disease
- D.
Chronic hepatitis
- E.
Duodenal ulcer
- F.
Acute pancreatitis
- G.
Chronic diarrhea
Correct Answer
E. Duodenal ulcerExplanation
The test described in the question is called the urea breath test. It is used to diagnose Helicobacter pylori infection, which is a common cause of duodenal ulcers. When the labeled urea is ingested, it is broken down by the bacterium, releasing labeled carbon dioxide. This labeled carbon dioxide is then detected in the breath samples, confirming the presence of H. pylori infection and duodenal ulcer. Therefore, the test is most likely part of the evaluation for duodenal ulcer.Rate this question:
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- 37.
A 4-year-old boy is brought to the physician by his mother because of decreased appetite and a fever of 100.4° F (38° C) for 2 days. His symptoms have also included abdominal cramps and diarrhea over the last 4 days, which were particularly alarming to the patient's mother when the stools became bloody. Physical examination reveals signs of mild dehydration and a mildly distended abdomen with diffuse pain elicited on palpation. Stool is sent for leukocytes, occult blood, and Gram stain and culture. Fecal leukocytes and occult blood tests are positive, and stool cultures grow non-lactose-fermenting gram-negative rods on MacConkey's agar. The organisms do not produce gas on fermentation of glucose and do not produce H,S on triple sugar iron (TSI) agar. Which of the following mechanisms is the most critical for this bacterium to cause disease?
- A.
Exotoxin production
- B.
Mucosal invasion
- C.
Intestinal colonization
- D.
Bacteremia
- E.
Proliferation in lymph nodes
Correct Answer
B. Mucosal invasionExplanation
The most critical mechanism for this bacterium to cause disease is mucosal invasion. This is supported by the patient's symptoms of abdominal cramps, diarrhea, and bloody stools, which indicate that the bacterium is able to invade and damage the mucosal lining of the gastrointestinal tract. The positive fecal leukocytes and occult blood tests further suggest an inflammatory response in the mucosa. Other options such as exotoxin production, intestinal colonization, bacteremia, and proliferation in lymph nodes may contribute to the disease process, but mucosal invasion is the primary mechanism responsible for the patient's symptoms.Rate this question:
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- 38.
An 8-year-old male is brought to the ER with a two day history of fever, abdominal pain and diarrhea. Careful history taking reveals that the patient's pet puppy had diarrhea one week ago. Aside from the present episode, the patient has no significant past medical history and all of his vaccinations are up-to-date. His stool is negative for ova and parasites. Which of the following is the most likely cause of this patient's symptoms?
- A.
Staphylococcus aureus
- B.
Shigella
- C.
Campylobacter
- D.
Bacillus cereus
- E.
Vibrio parahemolyticus
Correct Answer
C. CampylobacterExplanation
The most likely cause of this patient's symptoms is Campylobacter. The patient's history of fever, abdominal pain, and diarrhea, along with the fact that the patient's pet puppy had diarrhea one week ago, suggests an infectious cause. Campylobacter is a common cause of bacterial gastroenteritis, often transmitted through contaminated food or water, and can cause symptoms such as fever, abdominal pain, and diarrhea. Other options such as Staphylococcus aureus, Shigella, Bacillus cereus, and Vibrio parahemolyticus are less likely based on the given information.Rate this question:
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- 39.
A 5-month-old Hispanic boy is brought to the ER with complaints of poor feeding, weakness and complete loss of extremity muscle tone. All of his vaccinations are up to date and there is no significant past medical history. He receives formula as his sole source of nutrition with the exception of occasional fruit juice and honey. He has also received vitamin D supplementation. Which of the following tests is most likely to establish the diagnosis in this patient?
- A.
Stool for bacterial toxins
- B.
Blood for liver enzymes
- C.
Blood for viral titers
- D.
Urine for glucose and ketones
- E.
Urine for amino acids
Correct Answer
A. Stool for bacterial toxinsExplanation
The most likely diagnosis in this patient is botulism, a condition caused by the ingestion of the bacterial toxin produced by Clostridium botulinum. The symptoms of poor feeding, weakness, and loss of muscle tone are consistent with botulism. Stool for bacterial toxins is the most appropriate test to establish the diagnosis as it can detect the presence of the toxin in the patient's stool. Blood tests for liver enzymes or viral titers would not be helpful in this case. Urine tests for glucose and ketones or amino acids would also not provide relevant information for the diagnosis of botulism.Rate this question:
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- 40.
E co/iisolated from a patient with diarrhea does not ferment sorbitol on sorbitol containing Mac Conkey agar and does not produce glucuronidase. The toxin produced by these bacteria would most likely:
- A.
Activate adenylate cyclase
- B.
Activate guanylate cyclase
- C.
Inactivate ribosomal subunits
- D.
Inactivate EF-2
- E.
Disrupt the cellular cytoskeleton
Correct Answer
C. Inactivate ribosomal subunitsExplanation
The bacteria isolated from the patient with diarrhea do not ferment sorbitol and do not produce glucuronidase. This suggests that the bacteria are likely to be E. coli, specifically a strain known as E. coli O157:H7. This strain produces a toxin called Shiga toxin, which is known to inactivate ribosomal subunits. This toxin disrupts protein synthesis in the host cells by binding to the ribosomes and preventing them from functioning properly. This can lead to cell death and tissue damage, contributing to the symptoms of diarrhea.Rate this question:
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- 41.
E coli strains isolated from a 4-year-old Caucasian female with bloody diarrhea produce a substance that inhibits protein synthesis in human cells. The substance shares many properties with the toxin produced by:
- A.
Shigella dysenteriae
- B.
Salmonela typhi
- C.
Pseudomonas aeruginosa
- D.
Proteus mirabilis
- E.
Vibrio cholerae
- F.
Clostridium difficile
Correct Answer
A. Shigella dysenteriaeExplanation
The correct answer is Shigella dysenteriae because it is known to produce a toxin that inhibits protein synthesis in human cells, which is similar to the substance produced by the E. coli strains isolated from the 4-year-old with bloody diarrhea. Salmonella typhi, Pseudomonas aeruginosa, Proteus mirabilis, Vibrio cholerae, and Clostridium difficile do not produce a toxin with the same properties.Rate this question:
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- 42.
A 65-year-old male hospitalized for community acquired pneumonia develops fever and loose stools on his fourth hospital day. He recently traveled to Mexico for vacation without taking any prophylactic antibiotics or receiving any vaccinations. Sigmoidoscopy demonstrates white/yellow membrane-like plaques on the colonic mucosa and biopsy shows that these plaques are composed of fibrin and inflammatory cells. The patient's condition is most likely associated with which of the following?
- A.
Recent travel to Mexico
- B.
Missed vaccination
- C.
Antibiotic therapy
- D.
Gastritis prophylaxis
- E.
Consumption of home-canned foods
Correct Answer
C. Antibiotic therapyExplanation
The patient's condition is most likely associated with antibiotic therapy because the development of fever and loose stools after the fourth hospital day suggests the possibility of antibiotic-associated diarrhea. The presence of white/yellow membrane-like plaques on the colonic mucosa further supports this diagnosis, as it is consistent with pseudomembranous colitis, which is commonly caused by Clostridium difficile infection. This infection is a known complication of antibiotic therapy, as it disrupts the normal gut flora and allows for the overgrowth of C. difficile. The patient's recent travel to Mexico and consumption of home-canned foods are not directly related to the development of his symptoms.Rate this question:
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- 43.
Volunteer studies are used to determine the infective dose of Salmonella required to cause gastroenteritis. The curve obtained during the studies is shown on the diagram and is labeled as '2'. Which of the following organisms could be represented by line '1' on this diagram?
- A.
Shigella nexneri
- B.
Vibrio cholerae
- C.
Vibrio parahaemolyticus
- D.
Clostridium perfringens
- E.
Enterotoxigenic E coli
Correct Answer
A. Shigella nexneriExplanation
The question asks which organism could be represented by line '1' on the diagram. Since the diagram is used to determine the infective dose of Salmonella, we can infer that line '1' represents an organism that causes gastroenteritis. Among the given options, Shigella nexneri is the only organism known to cause gastroenteritis, so it is the most likely organism represented by line '1' on the diagram.Rate this question:
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- 44.
An outbreak of water-born gastroenteritis reported in Latin America is caused by oxidase-positive, gram-negative, comma-shaped rods that can survive on alkaline media. Stool microscopy in these patients is expected to demonstrate:
- A.
Mucus and some epithelial cells
- B.
Many leukocytes, neutrophils predominant
- C.
Many leukocytes, eosinophils predominant
- D.
Many leukocytes, lymphocytes predominant
- E.
Many erythrocytes and some leukocytes
Correct Answer
A. Mucus and some epithelial cellsExplanation
The outbreak of water-born gastroenteritis caused by oxidase-positive, gram-negative, comma-shaped rods indicates an infection with Vibrio cholerae. Stool microscopy in patients with cholera typically shows mucus and some epithelial cells. This is because the bacteria attach to the intestinal epithelial cells, causing them to slough off and appear in the stool. The presence of mucus indicates inflammation and damage to the intestinal lining, which is characteristic of cholera.Rate this question:
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- 45.
A 12-year-old boy is brought to the physician with fever, chills, and a rash that appeared this morning. Two days earlier, the child started complaining of a sore throat. His temperature is 38.3° C (101° F}. Examination shows a diffuse ery1hematous rash on his chest and abdomen that blanches with pressure, along with numerous 1- to 2-mm papules. The throat is ery1hematous with gray-white tonsillar exudates and the tongue is bright red. Which of the following is the most serious complication that can develop from this disease?
- A.
Aplastic anemia
- B.
Coronary artery aneurysm
- C.
Encephalitis
- D.
Orchitis
- E.
Rheumatic fever
Correct Answer
E. Rheumatic feverExplanation
The most serious complication that can develop from this disease is rheumatic fever. Rheumatic fever is a systemic inflammatory disease that can occur as a complication of untreated or inadequately treated streptococcal pharyngitis. It typically occurs 2-3 weeks after the initial infection and can affect the heart, joints, skin, and brain. It can lead to permanent damage to the heart valves, causing rheumatic heart disease, which can be life-threatening.Rate this question:
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- 46.
Non-pathogenic strains of Corynebacterium diphtheriae can acquire pathogenicity and thus the ability to cause severe pseudomembranous pharyngitis through which of the following mechanisms?
- A.
Transformation permitting capsule formation
- B.
Bacterial conjugation permitting pili expression
- C.
Phage conversion permitting exotoxin production
- D.
Chromosomal mutation permitting endotoxin production
- E.
Environmental influences permitting toxin synthesis
Correct Answer
C. Phage conversion permitting exotoxin productionExplanation
Phage conversion is a mechanism by which non-pathogenic strains of Corynebacterium diphtheriae can acquire pathogenicity. This occurs when a bacteriophage infects the non-pathogenic strain and introduces the gene for the production of the diphtheria exotoxin. The exotoxin is a key virulence factor that causes severe pseudomembranous pharyngitis, a characteristic symptom of diphtheria. Therefore, phage conversion is the mechanism by which non-pathogenic strains can become pathogenic and cause disease.Rate this question:
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- 47.
Clustered Gram positive bacteria are isolated from the tonsillar exudates of a 6-year-old boy. On microscopic examination, the bacteria have polar granules that stain deeply with aniline dyes. The pathogenicity of these organisms is the result of:
- A.
Widespread T-cell activation
- B.
Blockade of neurotransmitter release
- C.
Activation of electrolyte transport
- D.
Impairment of protein synthesis
- E.
Cellular membrane disruption
Correct Answer
D. Impairment of protein synthesisExplanation
The presence of polar granules that stain deeply with aniline dyes suggests that the bacteria in question are likely Corynebacterium diphtheriae, the causative agent of diphtheria. Diphtheria toxin, produced by C. diphtheriae, is responsible for the pathogenicity of these organisms. The toxin inhibits protein synthesis in host cells, leading to cell death and tissue damage. Therefore, the impairment of protein synthesis is the correct answer explaining the pathogenicity of these bacteria.Rate this question:
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- 48.
A 6-year-old immigrant from Eastern Europe is brought to the emergency department by his parents after they found him having difficulty breathing. The child has a fever and has not been eating or drinking for the last several hours. Head and neck examination reveals neck swelling, palatal paralysis, and a gray pharyngeal exudate. The parents are unable to provide information regarding the child's vaccination history. Which of the following interventions is most likely to improve this patient's prognosis?
- A.
Antibiotic therapy
- B.
Active immunization
- C.
Passive immunization
- D.
Adequate hydration
- E.
Anti-inflammatory medications
Correct Answer
C. Passive immunizationExplanation
Passive immunization is the most likely intervention to improve this patient's prognosis. The presentation of fever, difficulty breathing, neck swelling, palatal paralysis, and gray pharyngeal exudate suggests a possible diagnosis of diphtheria. Diphtheria is caused by the bacterium Corynebacterium diphtheriae, which produces a toxin that can lead to severe respiratory distress and other complications. Passive immunization with diphtheria antitoxin can provide immediate protection against the toxin and improve the patient's prognosis. Antibiotic therapy may also be necessary to eradicate the bacteria, but passive immunization is the most critical intervention in this case.Rate this question:
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- 49.
Three cases of severe pharyngitis were reported in a community of immigrants. The patients had thick pharyngeal exudates, neck swelling, and difficulty swallowing. One of them died from severe heart failure. The toxin responsible has a mechanism of action most similar to another toxin produced by which of the following bacteria?
- A.
Pseudomonas aeruginosa
- B.
Staphyllococcus aureus
- C.
Clostridium difficile
- D.
Clostridium botulinum
- E.
Bordetella pertussis
- F.
Vibrio cholerae
Correct Answer
A. Pseudomonas aeruginosaExplanation
The correct answer is Pseudomonas aeruginosa. Pseudomonas aeruginosa produces a toxin called exotoxin A, which has a mechanism of action similar to the toxin responsible for the severe pharyngitis in the patients. This toxin inhibits protein synthesis by ADP-ribosylation of elongation factor 2, leading to cell death. Staphylococcus aureus produces toxins such as toxic shock syndrome toxin-1 and exfoliative toxins, which have different mechanisms of action. Clostridium difficile produces toxins A and B, which cause pseudomembranous colitis. Clostridium botulinum produces botulinum toxin, which causes botulism. Bordetella pertussis produces pertussis toxin, which causes whooping cough. Vibrio cholerae produces cholera toxin, which causes cholera.Rate this question:
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- 50.
A 73-year-old woman comes to the physician complaining of progressive, severe pain and discharge from her left ear for the past 2 days. She has had type 2 diabetes for many years and has been noncompliant with her medications and follow-up appointments. On examination, moving or touching the pinna produces extreme pain. Otoscopic examination shows granulation tissue in the left ear canal with a scant amount of discharge. The tympanic membrane is clear, and there is no middle ear effusion. Initial cultures from the ear show a Gram-negative rod. Which of the following microbiological characteristics best describes the infecting organism?
- A.
Comma-shaped and grows well in high pH
- B.
Fast lactose fermenter
- C.
Motile and oxidase positive
- D.
Nonmotile and a lactose nonfermenter
- E.
Requires factors V and X for growth
Correct Answer
C. Motile and oxidase positiveExplanation
The infecting organism is described as motile and oxidase positive. This characteristic suggests that the organism is capable of movement and produces the enzyme oxidase. This information, along with the clinical presentation of pain and discharge in the ear, suggests that the patient may have an infection caused by Pseudomonas aeruginosa. Pseudomonas aeruginosa is a Gram-negative rod that commonly causes infections in immunocompromised individuals, such as those with poorly controlled diabetes. It is known to be motile and oxidase positive.Rate this question:
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