USMLE Microbiology Exam Practice Test

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  • 1/146 Questions

    Eight of 10 family practice residents who had a potluck four days ago now have diarrhea with abdominal cramps, general malaise, and fever ranging from 37.5 to 38.7 C (99.5 to 101.6 F). Stools from three residents are blood tinged. Laboratory studies revealed the causal agent was a microaerophilic gram-negative, curved rod with polar flagella often in pairs to give a "seagull" appearance. It grew on special media at 42 C (107.6 F). The original contami- nation probably was found in

    • Poultry
    • Improperly canned food
    • Fried rice
    • Fish
    • Vegetables
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About This Quiz

This USMLE Microbiology Exam Practice Test assesses key knowledge in medical microbiology, focusing on diseases, bacterial characteristics, and clinical scenarios. It prepares learners for real-world medical challenges and USMLE certification.

USMLE Microbiology Exam Practice Test - Quiz

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  • 2. 

    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:

    • Enrichment

    • Differential

    • Synthetic

    • Selective

    • Reducing

    Correct Answer
    A. Selective
    Explanation
    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.

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  • 3. 

    A 32-year-old migrant ranch worker is brought to the emergency room with painful paroxysmal involuntary muscle contractions involving mainly the jaw, neck and trunk. The causative substance has most likely traveled by which of the following routes?

    • Wound __, neuron axons __, salivary glands

    • Wound __, motor neuron axons __, spinal cord

    • Fibrinous exudate__, systemic circulation __, cortical neurons

    • Food__, systemic circulation__, meninges

    • Food __, systemic circulation __, peripheral nerves

    Correct Answer
    A. Wound __, motor neuron axons __, spinal cord
    Explanation
    The correct answer suggests that the causative substance has most likely traveled through a wound, then through motor neuron axons, and finally reached the spinal cord. This explanation indicates that the substance entered the body through a wound and then spread through the motor neuron axons, which are responsible for transmitting signals from the central nervous system to the muscles. The fact that the muscle contractions are mainly affecting the jaw, neck, and trunk supports the idea that the substance has reached the spinal cord, which controls these muscle groups.

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  • 4. 

    A cancer chemotherapy patient has to have her intravenous port revised after it becomes blocked and the catheter is found to contain bacterial con- taminants. Which of the following attributes is most likely to be a factor in this pathogenesis?

    • Biofilm production

    • Ergosterol containing membrane

    • Peptidoglycan layer

    •  Possession of IgA protease

    • Possession of pili

    Correct Answer
    A. Biofilm production
    Explanation
    Biofilm production is most likely to be a factor in the pathogenesis of the blocked intravenous port and bacterial contamination in the catheter. Biofilms are communities of bacteria that attach to surfaces and form a protective matrix. They are known to be highly resistant to antibiotics and immune responses, making them difficult to eradicate. In this case, the biofilm formed by the bacteria in the catheter could have caused the blockage and also provided a protected environment for bacterial growth, leading to contamination.

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  • 5. 

    What is the typical means of transmission of a toxin that blocks the release of inhibitory transmitters GABA and glycine?

    • Eating home-canned foods

    • Fecal-oral, travel to foreign country

    • Infant given honey during the first year of life

    • Puncture wound

    • Respiratory, with incomplete vaccination history

    Correct Answer
    A. Puncture wound
    Explanation
    A puncture wound can be a typical means of transmission for a toxin that blocks the release of inhibitory transmitters GABA and glycine. When a puncture wound occurs, it can allow the entry of toxins into the body, which can then affect the release of these inhibitory transmitters. This can disrupt the normal functioning of the nervous system and lead to various symptoms and complications. Therefore, a puncture wound can be a potential route for the transmission of such toxins.

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  • 6. 

    A patient is admitted to the hospital because of a bleeding duodenal ulcer. Culture at 37.0 C (98.6 F) reveals a urease-positive, gram-negative, curved rod. Which of the following is a likely complication due to infection with the causal agent?

    • Diarrhea

    • Kidney stones

    •  Pseudomembranous colitis

    • Stomach cancer

    • Vomiting

    Correct Answer
    A. Stomach cancer
    Explanation
    Infection with the causal agent of a bleeding duodenal ulcer, which is a urease-positive, gram-negative, curved rod, can lead to stomach cancer as a likely complication. This is because chronic infection with certain bacteria, such as Helicobacter pylori, can cause inflammation and damage to the lining of the stomach, increasing the risk of developing stomach cancer over time.

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  • 7. 

    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:

    • Coagulase positive

    • Hemolytic

    • Novobiocin resistant

    • DNase positive

    • Yellow pigment producer

    Correct Answer
    A. Novobiocin resistant
    Explanation
    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.

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  • 8. 

    An 18-year-old college freshman is brought to the emergency room with a high fever, confusion and headaches. Physical examination reveals nuchal rigidity and a purpuric rash on his lower extremities. This infection could have been prevented by a vaccine containing:

    • Heat-killed bacteria

    • Bacterial outer membrane protein

    • Capsular polysaccharide

    • Inactivated toxin

    • Live attenuated bacteria

    Correct Answer
    A. Capsular polysaccharide
    Explanation
    The correct answer is capsular polysaccharide. This is because the patient's symptoms, including fever, confusion, headaches, nuchal rigidity, and purpuric rash, are consistent with meningococcal meningitis, which is caused by the bacterium Neisseria meningitidis. The capsule of this bacterium is composed of polysaccharides, and vaccines containing capsular polysaccharide have been shown to be effective in preventing meningococcal infections.

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  • 9. 

    A 58-year-old male presents to the ER with fever, headache and vomiting. His past medical history is significant for a recent kidney transplantation. Lumbar puncture reveals CSF pleocytosis and a normal CSF glucose concentration. Gram positive rods with tumbling motility at room temperature are seen on CSF microscopy. How was this patient's infection most likely transmitted?

    • Unprotected sex

    • Contaminated food

    • Respiratory droplets

    • Arthropod bite

    • Needle stick

    Correct Answer
    A. Contaminated food
    Explanation
    This patient's infection is most likely transmitted through contaminated food. The presence of gram positive rods with tumbling motility at room temperature in the CSF indicates infection with Listeria monocytogenes, which is commonly transmitted through the ingestion of contaminated food, especially unpasteurized dairy products, deli meats, and soft cheeses. This mode of transmission is consistent with the patient's symptoms of fever, headache, and vomiting.

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  • 10. 

    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?

    • Mast cells and eosinophils

    • Basophils and macrophages

    • Neutrophils and B lymphocytes

    • Macrophages and T lymphocytes

    • Platelets and mast cells

    Correct Answer
    A. Macrophages and T lymphocytes
    Explanation
    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.

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  • 11. 

    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:

    • Methotrexate therapy

    • Proper sore throat treatment

    • Timely vaccination

    • Barrier contraception

    • Avoiding intravenous drug abuse

    Correct Answer
    A. Barrier contraception
    Explanation
    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.

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  • 12. 

    A developing country includes infant vaccination with the Haemophi/us inffuenzaetype b conjugate vaccine into its routine immunization schedule. If effectively implemented, this change in the vaccination schedule would most likely affect the epidemiology of:

    • Otitis media

    • Sinusitis

    • Acute bronchitis

    • Meningitis

    • Conjunctivitis

    Correct Answer
    A. Meningitis
    Explanation
    The introduction of the Haemophilus influenzae type b (Hib) conjugate vaccine into the routine immunization schedule of a developing country would most likely affect the epidemiology of meningitis. This is because Hib is one of the leading causes of bacterial meningitis, especially in children under the age of 5. By vaccinating infants against Hib, the country would significantly reduce the number of Hib infections and consequently decrease the incidence of meningitis caused by this bacterium.

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  • 13. 

    Bacteria isolated from the lung tissue of a 32-year-old Caucasian male fail to decolorize with hydrochloric acid and alcohol after staining carbolfuchsin. Which of the following cell wall components is most likely responsible for this staining phenomenon?

    • N-acetylmuramic acid

    • Teichoic acid

    • Lipopolysaccharide

    • Mycolic acid

    • Ergosterol

    Correct Answer
    A. Mycolic acid
    Explanation
    Mycolic acid is a component found in the cell wall of acid-fast bacteria, such as Mycobacterium tuberculosis. It is responsible for the staining phenomenon observed in acid-fast staining methods, such as the Ziehl-Neelsen stain, where bacteria retain the carbolfuchsin stain even after decolorization with acid and alcohol. This characteristic staining property of mycolic acid is due to its high lipid content and resistance to decolorization. Therefore, the failure of the bacteria isolated from the lung tissue to decolorize with hydrochloric acid and alcohol after staining with carbolfuchsin suggests the presence of mycolic acid in their cell wall.

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  • 14. 

    A 5-year-old boy is brought to the ER with breathing difficulty, dysphagia, drooling and fever. His temperature is 39.4C (103.0 F). White blood cell count is 23,000/cmm with many band forms. Laryngoscopy in the operating room shows a swollen and cherry-red epiglottis. This patient most likely:

    • Traveled recently

    • Missed vaccination

    • Was stung by a bee

    • Has penicillin allergy

    • Has characteristic family history

    Correct Answer
    A. Missed vaccination
    Explanation
    The correct answer is "Missed vaccination" because the patient's symptoms are consistent with acute epiglottitis, which is most commonly caused by Haemophilus influenzae type B (Hib) infection. Hib vaccination is part of routine childhood immunization and has greatly reduced the incidence of epiglottitis. Therefore, a missed vaccination would increase the risk of developing this condition. The symptoms of breathing difficulty, dysphagia, drooling, and fever, along with the physical finding of a swollen and cherry-red epiglottis, support this diagnosis.

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  • 15. 

    A 78-year-old nursing home resident develops high fevers, chest pain and cough productive of rusty sputum. Sputum microscopy reveals numerous lancet-shaped Gram positive cocci in pairs. The patient dies despite aggressive hydration and antibiotic treatment. A vaccine containing which of the following might have prevented this outcome?

    • Recombinant surface protein

    • Capsular polysaccharide

    • Live attenuated bacteria

    • Killed bacteria

    • Inactivated toxin

    • Synthetic oligopeptides

    Correct Answer
    A. Capsular polysaccharide
    Explanation
    Capsular polysaccharide is the correct answer because the patient's symptoms and sputum microscopy findings are consistent with pneumococcal pneumonia, which is caused by Streptococcus pneumoniae. The capsule of S. pneumoniae is composed of polysaccharides, and it helps the bacteria evade the immune system. Vaccines containing capsular polysaccharides from different strains of S. pneumoniae have been developed and have been shown to be effective in preventing pneumococcal infections, including pneumonia. Therefore, a vaccine containing capsular polysaccharide might have prevented the outcome of the patient's death.

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  • 16. 

    A sample of contaminated moist soil is heated to 1 oo• C for 15 minutes. Which of the following bacteria is most likely to be recovered from the soil sample following heat exposure?

    • Streptococcus pyogenes

    • Listeria monocytogenes

    • Escherichia coli

    • Bacillus anthracis

    • Brucella melitensis

    Correct Answer
    A. Bacillus anthracis
    Explanation
    Bacillus anthracis is the most likely bacteria to be recovered from the soil sample following heat exposure because it is a spore-forming bacteria. Spore-forming bacteria have the ability to form dormant spores that can withstand extreme conditions, including high temperatures. Heating the soil to 100°C for 15 minutes would kill most bacteria, but spores can survive and later germinate when conditions become favorable again. Bacillus anthracis is known to form highly resistant spores, making it more likely to survive the heat exposure compared to the other bacteria listed.

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  • 17. 

    A 55-year-old woman had her rheumatic heart valve replaced with a pros- thetic valve. Six blood cultures became positive after three days of incuba- tion. An optochin-resistant, catalase-negative gram-positive coccus that was alpha-hemolytic was isolated. What was the most likely causal agent?

    • Streptococcus viridans

    • Pseudomonas aeruginosa

    • Serratia marcescens

    • Staphylococcus aureus

    • Streptococcus pneumoniae

    Correct Answer
    A. Streptococcus viridans
    Explanation
    The most likely causal agent in this case is Streptococcus viridans. This is indicated by the fact that the isolated organism is a gram-positive coccus that is alpha-hemolytic, optochin-resistant, and catalase-negative. Streptococcus viridans is known to cause infective endocarditis in patients with prosthetic heart valves, making it the most likely explanation in this scenario.

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  • 18. 

    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?

    • Endometrial hyperplasia

    • Hydatidiform mole

    • Ovarian cancer

    • Polycystic ovary syndrome

    • Infertility

    Correct Answer
    A. Infertility
    Explanation
    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.

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  • 19. 

    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?

    • Klebsiella pneumoniae

    • Samonella typhi

    • Salmonella enteritidis

    • Campylobacter jejuni

    • Shigella dysenteriae

    • Proteus mirabilis

    • Yersinia enterocolitica

    • Haemophilus influenzae

    • Vibrio cholerae

    • Streptococcus pyogenes

    • Streptococcus pneumoniae

    • Escherichia coli

    Correct Answer
    A. Escherichia coli
    Explanation
    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.

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  • 20. 

    A 3-year-old male experiences several episodes of otitis media. Middle ear exudate cultures grow H. inffuenzae although the patient's immunizations are up-to-date. Which of the following explains H. inffuenzae infection in this patient despite immunizations?

    • No vaccine is effective against H. inffuenzae

    • The strains responsible for this patient's disease produce exotoxin

    • The strains responsible for this patient's disease do not produce capsule

    • H. inffuenzae infection despite immunizations means defective cellular immunity

    • H. inffuenzae infection despite immunizations signifies defective neutrophil function

    Correct Answer
    A. The strains responsible for this patient's disease do not produce capsule
    Explanation
    The correct answer is that the strains responsible for this patient's disease do not produce a capsule. The capsule of H. influenzae is an important virulence factor that allows the bacteria to evade the immune system and cause infection. Without a capsule, the bacteria are more susceptible to the immune response and are less likely to cause disease. Therefore, even though the patient's immunizations are up-to-date, they are still susceptible to infection with non-capsulated strains of H. influenzae.

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  • 21. 

    A 45-year-old female executive goes to a cosmetic surgeon with the com- plaint of frown lines on her forehead which she feels are negatively affecting her appearance. Rather than undergoing surgery, she opts to try injection of Botox. What is the mechanism of action of this toxin?

    • It blocks release of acetylcholine.

    • It inhibits glycine and GABA.

    • It is a lecithinase.

    •  It is a superantigen.

    • It ribosylates eukaryotic elongation factor-2.

    •  It ribosylates Gs.

    Correct Answer
    A. It blocks release of acetylcholine.
    Explanation
    Botox works by blocking the release of acetylcholine, a neurotransmitter responsible for muscle contraction. By inhibiting the release of acetylcholine, Botox temporarily paralyzes the muscles, preventing them from contracting and thus reducing the appearance of frown lines on the forehead. This mechanism of action is what allows Botox to be used as a cosmetic treatment for wrinkles and lines on the face.

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  • 22. 
    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? - ProProfs

    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?

    • Shigella nexneri

    • Vibrio cholerae

    • Vibrio parahaemolyticus

    • Clostridium perfringens

    • Enterotoxigenic E coli

    Correct Answer
    A. Shigella nexneri
    Explanation
    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.

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  • 23. 

    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:

    • Antigenic mimicry

    • Low molecular weight

    • Non-protein structure

    • Antigenic variation

    • Local immunosuppression

    Correct Answer
    A. Antigenic variation
    Explanation
    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.

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  • 24. 

    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?

    • Escherichia coli

    • Pseudomonas aeruginosa

    • Klebsiella pneumoniae

    • Staphylococcus saprophyticus

    • Enterococcus cloacae

    • Ureaplasma urealyticum

    Correct Answer
    A. Pseudomonas aeruginosa
    Explanation
    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.

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  • 25. 

    A 54-year-old male presents to the ER with a sore on his right shoulder. Physical examination demonstrates an ulcer with central black eschar surrounded by edema. Exudate microscopy reveals chains of large Gram-positive rods. Which of the following would be most helpful in making the diagnosis?

    • Family history

    • Occupation

    • Recent contact with foreigners

    • Allergies

    • Current medications

    • Sexual history

    • Prior ER visits

    Correct Answer
    A. Occupation
    Explanation
    Occupation would be most helpful in making the diagnosis because the presentation of an ulcer with central black eschar surrounded by edema and the finding of chains of large Gram-positive rods on exudate microscopy suggests a diagnosis of cutaneous anthrax. Cutaneous anthrax is most commonly seen in individuals who work with animals or animal products, such as farmers, veterinarians, or butchers. Therefore, knowing the patient's occupation would provide important information to support the diagnosis of cutaneous anthrax.

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  • 26. 

    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?

    • Dental extraction

    • Skin biopsy

    • Sinus drainage

    • Nasal polypectomy

    • Cystoscopy

    Correct Answer
    A. Cystoscopy
    Explanation
    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.

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  • 27. 

    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:

    • Prevent viral attachment to target cells

    • Inhibit viral protease activity

    • Incorporate into newly replicated viral DNA

    • Prevent virion release from infected cells

    • Up-regulate interferon synthesis in infected cells

    Correct Answer
    A. Incorporate into newly replicated viral DNA
    Explanation
    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.

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  • 28. 

    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?

    • Joint pain and eye redness

    • Fatigue and heart murmurs

    • Face puffiness and dark urine

    • Bilateral facial nerve palsy

    • Abdominal pain and jaundice

    Correct Answer
    A. Face puffiness and dark urine
    Explanation
    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.

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  • 29. 

    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?

    • Exotoxin formed prior to ingestion

    • Exotoxin formed after ingestion

    • Endotoxin

    • Bacterial invasion of gut mucosa

    • Parasitic infection

    Correct Answer
    A. Exotoxin formed prior to ingestion
    Explanation
    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.

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  • 30. 

    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:

    • Widespread T-cell activation

    • Blockade of neurotransmitter release

    • Activation of electrolyte transport

    • Impairment of protein synthesis

    • Cellular membrane disruption

    Correct Answer
    A. Impairment of protein synthesis
    Explanation
    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.

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  • 31. 

    Health authorities are investigating an outbreak of respiratory infections among a group of military recruits. Fifteen recruits have reported persistent cough, low-grade fever and malaise. Each patient's chest X-ray shows pulmonary infiltrates more severe in appearance than would be expected from the patient's clinical status. Sputum cultures require a complex acellular medium enriched with cholesterol in order to grow. Which of the following organisms is the most likely cause of the outbreak?

    • Streptococcus pneumoniae

    • Klebsiella pneumoniae

    • Haemophilus inffuenzae

    • Legionella pneumophila

    • Mycoplasma pneumoniae

    Correct Answer
    A. Mycoplasma pneumoniae
    Explanation
    The most likely cause of the outbreak is Mycoplasma pneumoniae. This is suggested by the symptoms reported by the recruits, such as persistent cough, low-grade fever, and malaise, which are consistent with a respiratory infection. The chest X-rays showing pulmonary infiltrates that are more severe than expected also align with Mycoplasma pneumoniae, as it is known to cause atypical pneumonia with extensive lung involvement. Additionally, the fact that sputum cultures require a complex acellular medium enriched with cholesterol to grow further supports Mycoplasma pneumoniae as the likely organism, as it is a fastidious bacterium that requires specific growth conditions.

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  • 32. 

    A 7-day-old infant presents to the emergency department with a fever, poor feeding, and a bulging fontanelle. During her physical examination, she begins to convulse. A Gram stain of the CSF reveals gram-positive rods. Which of the following organisms is the most likely causal agent?

    • Escherichia coli

    • Haemophilus influenzae

    • Listeria monocytogenes

    • Neisseria meningitidis

    • Streptococcus agalactiae

    Correct Answer
    A. Listeria monocytogenes
    Explanation
    Listeria monocytogenes is the most likely causal agent in this case because it is a gram-positive rod and is known to cause meningitis in neonates. The presentation of fever, poor feeding, bulging fontanelle, and convulsions is consistent with meningitis. Listeria monocytogenes is a common cause of bacterial meningitis in infants less than 1 month old, especially in premature infants. It can be transmitted to the infant through the placenta or during delivery. Prompt treatment with antibiotics is essential to prevent serious complications and mortality.

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  • 33. 

    A 15-day-old boy presents with conjunctivitis. Iodine staining bodies are seen in conjunctival scrapings. The most likely infectious form is a(n)

    • Elementary body

    • Reticulate body

    • Endospore

    • Exotoxin

    • Vegetative cell

    Correct Answer
    A. Elementary body
    Explanation
    The presence of iodine staining bodies in conjunctival scrapings suggests the presence of elementary bodies. Elementary bodies are the infectious form of certain bacteria, such as Chlamydia trachomatis, which causes conjunctivitis. These small, round bodies are resistant to environmental conditions and can survive outside of host cells. They are responsible for initiating infection and are typically found in the initial stages of the disease. Reticulate bodies, endospores, exotoxins, and vegetative cells are not typically associated with conjunctivitis caused by Chlamydia trachomatis.

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  • 34. 

    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?

    • Decreased thickness of the vaginal epithelium

    • Decreased glycogen concentration in the vaginal epithelium

    • Decreased number of Gram-positive bacteria in the vagina

    • Increased alkaline secretions by the cervical mucosa

    • Depressed T-lymphocyte response to mucosal stimuli

    Correct Answer
    A. Decreased number of Gram-positive bacteria in the vagina
    Explanation
    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.

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  • 35. 

    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:

    • Intestinal invasion

    • 'Stacked-brick' intestinal adhesion

    • Shiga-like toxin

    • Cholera-like toxin

    • Endotoxin release

    Correct Answer
    A. Cholera-like toxin
    Explanation
    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.

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  • 36. 

    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?

    • Patients previously exposed to the bacteria

    • Patients with chronic pancreatitis

    • Patients on omeprazole treatment

    • Patients with chronic hepatitis

    • Patients with peptic ulcer disease

    Correct Answer
    A. Patients on omeprazole treatment
    Explanation
    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.

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  • 37. 

    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?

    • Stool for bacterial toxins

    • Blood for liver enzymes

    • Blood for viral titers

    • Urine for glucose and ketones

    • Urine for amino acids

    Correct Answer
    A. Stool for bacterial toxins
    Explanation
    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.

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  • 38. 

    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?

    • Aplastic anemia

    • Coronary artery aneurysm

    • Encephalitis

    • Orchitis

    • Rheumatic fever

    Correct Answer
    A. Rheumatic fever
    Explanation
    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.

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  • 39. 

    Autopsy of a 14-year-old male who died of septic shock shows extensive hyperemia and infarcts of his internal organs. Purpuric cutaneous lesions and hemorrhagic destruction of both adrenals are also present. The organism most likely responsible for this patient's death more commonly causes which of the following?

    • Urinary tract infection

    • Pneumonia

    • Infectious diarrhea

    • Endocarditis

    • Meningitis

    • Skin infection

    Correct Answer
    A. Meningitis
    Explanation
    The autopsy findings of hyperemia and infarcts of internal organs, purpuric cutaneous lesions, and hemorrhagic destruction of both adrenals suggest disseminated intravascular coagulation (DIC). DIC is often caused by meningococcal meningitis, which can lead to septic shock and multiorgan failure. Therefore, the organism most likely responsible for this patient's death, meningococcus, commonly causes meningitis.

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  • 40. 

    A group of investigators has developed monoclonal antibodies to the meningococcal pilus. They believe that these antibodies may decrease the risk of infection by meningococci. If effective, these antibodies would directly interfere with which of the following processes?

    • Intestinal attachment

    • Endothelial adhesion

    • Pharyngeal colonization

    • Incomplete phagocytosis

    • Hematogenous dissemination

    Correct Answer
    A. Pharyngeal colonization
    Explanation
    Monoclonal antibodies to the meningococcal pilus would directly interfere with pharyngeal colonization. This means that the antibodies would prevent the bacteria from colonizing or establishing an infection in the pharynx, which is the area at the back of the throat. By targeting and inhibiting this colonization process, the antibodies would decrease the risk of infection by meningococci.

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  • 41. 

    A county hospital experiences an outbreak of methicillin-resistant Staphylococcus aureus (MRSA) wound infections. You suspect that many staff members are carriers of the bacteria. Which of the following would you expect to be the most commonly MRSA-populated site in staff members?

    • Hands

    • Axilla

    • Anterior nares

    • Perineum

    • Oropharynx

    Correct Answer
    A. Anterior nares
    Explanation
    The most commonly MRSA-populated site in staff members would be the anterior nares. This is because the anterior nares, or the nostrils, are a common colonization site for Staphylococcus aureus, including the methicillin-resistant strains. The bacteria can easily colonize the anterior nares and be transmitted through direct contact or respiratory droplets. Therefore, it is likely that staff members who are carriers of MRSA would have the bacteria present in their anterior nares.

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  • 42. 

    A 46-year-old female presents to the emergency room with fever, chest pain and hemoptysis. She came to visit her family from Pakistan where she works in a goat wool processing center. Chest x-ray shows widened mediastinum. Sputum and blood cultures reveal large gram-positive rods that form 'medusa head' colonies on standard media. The bacteria isolated from this patient most likely produce:

    • Antiphagocytic D-glutamate capsule

    • Antiphagocytic polysaccharide capsule

    • LgG-binding outer membrane protein

    • Intracellular polyphosphate granules

    • Peritrichous flagella

    Correct Answer
    A. Antiphagocytic D-glutamate capsule
    Explanation
    The correct answer is "Antiphagocytic D-glutamate capsule". This is because the patient's symptoms, including fever, chest pain, and hemoptysis, along with the widened mediastinum on chest x-ray, are consistent with a diagnosis of anthrax. The presence of large gram-positive rods that form "medusa head" colonies on standard media further supports this diagnosis. Anthrax is caused by the bacteria Bacillus anthracis, which is known to produce an antiphagocytic D-glutamate capsule. This capsule helps the bacteria evade the immune system and contributes to its pathogenicity.

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  • 43. 

    A 10-year-old Caucasian male is brought to the ER with a several day history of high fevers and chills. He also complains of dull pain just above the left knee. There is no joint effusion. X-ray films show soft tissue swelling, bone destruction, and periosteal reaction over the lower end of femur. Which of the following organisms is most likely responsible for this patient's symptoms?

    • Staphylococcus aureus

    • Staphylococcus epidennidis

    • Streptococcus pyogenes

    • Streptococcus agalactiae

    • Streptococcus pneumoniae

    • Streptococcus taecalis

    Correct Answer
    A. Staphylococcus aureus
    Explanation
    The patient's symptoms, including high fevers, chills, and pain above the left knee, along with the findings on X-ray films of soft tissue swelling, bone destruction, and periosteal reaction, are indicative of osteomyelitis. Staphylococcus aureus is the most common causative organism for acute hematogenous osteomyelitis, especially in children. Therefore, Staphylococcus aureus is the most likely responsible organism for this patient's symptoms.

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  • 44. 

    What is the structure that is found in gram-negative but not in gram- positive bacteria?

    • Capsule

    • Cell wall

    • Cytoplasmic membrane

    • Endospore

    • Outer membrane

    Correct Answer
    A. Outer membrane
    Explanation
    Gram-negative bacteria have an outer membrane, which is an additional layer outside the cell wall. This outer membrane is absent in gram-positive bacteria. The outer membrane in gram-negative bacteria provides an extra barrier of protection and helps to prevent the entry of certain substances, such as antibiotics, into the cell. It also contains specific proteins called porins, which allow the passage of certain molecules. This structural difference between gram-negative and gram-positive bacteria is important in understanding their different responses to antibiotics and other substances.

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  • 45. 

    A 65-year-old man develops pneumonia. The organisms isolated from the sputum are gram-positive cocci that are alpha hemolytic on blood agar and sensitive to optochin. Which structure of the causal agent provides protec- tion against phagocytosis?

    • Capsule

    • Catalase

    • Coagulase

    • M protein

    • Teichoic acid

    Correct Answer
    A. Capsule
    Explanation
    The correct answer is Capsule. The presence of a capsule provides protection against phagocytosis by preventing the immune system cells from engulfing the bacteria. The capsule acts as a barrier, making it difficult for phagocytes to recognize and engulf the bacteria. This allows the bacteria to evade the immune system and survive in the host, leading to the development of pneumonia.

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  • 46. 

    A 38-year-old man who recently visited India on business presents to the emergency department with profuse watery diarrhea flecked with mucus, and severe dehydration. Which of the following correctly describes the causal agent?

    • Gram-negative curved rod; toxin that increases cAMP

    • Gram-negative curved rod; toxin that inhibits protein synthesis

    • Gram-negative rod; toxin that increases cAMP

    • Gram-negative rod; toxin that inhibits protein synthesis

    • Intoxication with a heat labile toxin that blocks the release of acetylcholine

    Correct Answer
    A. Gram-negative curved rod; toxin that increases cAMP
    Explanation
    The correct answer describes the causal agent as a Gram-negative curved rod that produces a toxin which increases cAMP levels. This is consistent with the presentation of profuse watery diarrhea flecked with mucus, which is characteristic of cholera caused by Vibrio cholerae. The toxin produced by Vibrio cholerae, called cholera toxin, increases cAMP levels in the intestinal epithelial cells, leading to the secretion of large amounts of fluid into the intestinal lumen and subsequent severe dehydration.

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  • 47. 

    A 70-year-old man is hospitalized for an infection and treated with clindamy- cin. The patient improves and returns to his nursing home. Two weeks later he is rushed to the emergency room with fever and loose, mucoid green stools. The diarrhea is voluminous, and he is having severe abdominal pain. Sigmoidoscopy of his colon reveals yellow-white plaques. What is the single most likely event/factor that contributed to this patient's current illness?

    •  Administration of antibiotics

    • Advanced age

    • Drinking unpasteurized milk

    • Eating contaminated cold cuts

    •  Living in nursing home

    Correct Answer
    A.  Administration of antibiotics
    Explanation
    The administration of antibiotics is the single most likely event/factor that contributed to this patient's current illness. This is because the patient was treated with clindamycin, which is known to disrupt the normal flora of the gut and can lead to an overgrowth of Clostridium difficile. This overgrowth can cause a severe infection known as Clostridium difficile-associated diarrhea (CDAD), which is characterized by fever, loose mucoid green stools, voluminous diarrhea, and severe abdominal pain. The yellow-white plaques seen on sigmoidoscopy are consistent with pseudomembranous colitis, a complication of CDAD.

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  • 48. 

    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?

    • Gardnerella vaginalis

    • Neisseria gonorrhoeae

    • Trichomonas vaginalis

    • Treponema pallidum

    • Herpes simplex virus

    • Molluscum contagiosum

    Correct Answer
    A. Neisseria gonorrhoeae
    Explanation
    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.

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  • 49. 

    A 35-year-old man presents to your office complaining of a hard mass under his jaw. He reports that the mass developed three months ago, shortly after he had a tooth extracted. The mass has since grown in size, and has recently begun to drain yellowish pus through the overlying skin. Which of the following is the most likely cause of this patient's condition?

    • Borrelia burgdorferi

    • Pasteurella multocida

    • Sporothrix schenckii

    • Actinomyces israelii

    • Bacillus anthracis

    • Rickettsia rickettsiae

    Correct Answer
    A. Actinomyces israelii
    Explanation
    The most likely cause of the patient's condition is Actinomyces israelii. Actinomyces is a gram-positive bacteria that can cause chronic infections in the head and neck region. The patient's history of tooth extraction and the development of a hard mass under his jaw, which has grown in size and is now draining pus, are consistent with actinomycosis. Actinomyces israelii is known to cause this type of infection, which often presents as a firm, non-tender mass with sinus tract formation and purulent drainage.

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  • Jan 20, 2023
    Quiz Edited by
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  • Dec 29, 2020
    Quiz Created by
    Catherine Halcomb

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