USMLE Microbiology Exam Practice Test

The selective medium containing vancomycin, colistin, nystatin, and trimethoprim is specifically designed to favor the growth of Neisseria gonorrhoeae. This bacterium is the causative agent of gonorrhea, a sexually transmitted infection that commonly affects the pharynx. By using this selective medium, the researchers can isolate and identify asymptomatic carriers of N. gonorrhoeae in the 20-25-year-old age group. The other bacteria listed are not specifically targeted by this selective medium and are less likely to grow under these conditions.

The correct answer is people because the question states that the residents in the retirement home have taken their meals prepared by the same kitchen. Since blood cultures from 11 of these residents grow Salmonella enterica subsp. typhi, it suggests that the primary reservoir of this organism is people. This means that the residents themselves are the source of the Salmonella infection, possibly due to poor hygiene or contaminated food handling practices.

The correct answer is Salmonella enterica subsp. typhi. This is the most likely cause of the tourist's condition based on the symptoms described. The persistent high fever, malaise, constipation, enlarged spleen, and rose-colored spots are all consistent with typhoid fever, which is caused by Salmonella enterica subsp. typhi. The isolation of a bacterium that produces H2S and is motile further supports this diagnosis, as these are characteristic features of Salmonella enterica subsp. typhi.

The most likely causal agent in this case is Enterococcus faecalis. This is because the question mentions that the organisms isolated are gram-positive cocci that are catalase-negative and capable of growth in 6.5% sodium chloride. Enterococcus faecalis fits this description as it is a gram-positive cocci bacterium that is catalase-negative and can grow in high salt concentrations. Additionally, Enterococcus faecalis is known to cause urinary tract infections and is commonly associated with hospital-acquired infections.

The description of the causal agent is "Gram-positive coccus, beta hemolytic, catalase negative." This means that the organism responsible for the girl's symptoms is a type of bacteria that appears purple when stained with the Gram stain, is able to completely break down red blood cells (beta hemolysis), and does not produce the enzyme catalase. This information helps to narrow down the possible bacterial causes of her symptoms and guide treatment decisions.

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.

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.

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.

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.

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.

Cysteine-tellurite agar would facilitate the growth of the organism causing the symptoms in the 6-year-old male. Cysteine-tellurite agar is a selective medium used for the isolation and identification of Corynebacterium diphtheriae, the bacteria responsible for diphtheria. The grey pharyngeal exudate and the presence of scant Gram positive organisms on the Gram stain are consistent with a possible diphtheria infection. Cysteine-tellurite agar contains cysteine, which enhances the growth of C. diphtheriae, and tellurite, which inhibits the growth of other bacteria. Therefore, this medium would be the most appropriate for isolating and identifying the causative organism in this case.

The presence of nuchal rigidity and the patient's symptoms of headache and nausea suggest that he may have meningitis. Gram stain of the cerebrospinal fluid (CSF) can provide important information about the causative organism. In this case, the most likely finding on Gram stain would be lancet-shaped, Gram-positive cocci in pairs. This is consistent with the most common cause of bacterial meningitis, which is Streptococcus pneumoniae.

The conjugated diphtheria toxoid to the polysaccharide in this vaccine increases immunogenicity. This means that it enhances the ability of the vaccine to stimulate an immune response in the body. By combining the polysaccharide with the diphtheria toxoid, the vaccine is able to elicit a stronger immune response, leading to a more effective protection against H. influenzae.

Pertussis toxin is responsible for the production of lymphocytosis, insulin secretion, and histamine sensitization in this organism.

The correct answer is M proteins. Glomerulonephritis is often caused by a group A Streptococcus infection, which can occur after a sore throat. The M proteins are a major virulence factor of group A Streptococcus and are responsible for the development of glomerulonephritis. These proteins help the bacteria evade the immune system and can also cross-react with host tissues, leading to an autoimmune response and damage to the kidneys. Therefore, the identification of M proteins through serotyping can help determine the causal agent in this case.

The correct answer is "Gram-positive aerobic rod; toxin that inhibits protein synthesis." This is because the description of the causal agent matches that of Corynebacterium diphtheriae, which is a Gram-positive aerobic rod. Corynebacterium diphtheriae produces diphtheria toxin, which inhibits protein synthesis in host cells, leading to the characteristic symptoms of diphtheria such as sore throat, malaise, and difficulty breathing. The presence of cervical lymphadenopathy and a gray, leathery exudate in the oropharynx further supports this diagnosis.

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.

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.

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.

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.

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.

The correct answer is home-canned food consumption. The patient's symptoms of difficulty swallowing and blurred vision of sudden onset suggest the possibility of botulism, which is caused by the ingestion of the botulinum toxin produced by Clostridium botulinum bacteria. Home-canned foods are a common source of botulism toxin, as the bacteria can grow in improperly canned foods. Therefore, it is important to question the patient about their consumption of home-canned foods to assess the likelihood of botulism as the cause of their symptoms.

The patient's symptoms of progressive difficulty walking, tingling in the feet, decreased muscular strength, and absent deep tendon reflexes are consistent with Guillain-Barré syndrome (GBS). GBS is an autoimmune condition that often occurs after an infection, and Campylobacter is one of the most common bacterial causes of GBS. The patient's history of diarrhea two weeks ago suggests an infection, and Campylobacter is a common cause of bacterial gastroenteritis. Therefore, Campylobacter is the most likely agent associated with the patient's condition.

The bacteria isolated from the injection site assume a spherical configuration when placed in an isotonic solution and disintegrate rapidly when placed in a hypotonic solution. This suggests that the bacteria have a thick peptidoglycan layer in their cell wall, which is a characteristic of Gram-positive bacteria. Cefuroxime is a type of antibiotic that targets the cell wall synthesis of bacteria, making it effective against Gram-positive bacteria. Therefore, cefuroxime is the most likely antibiotic used in this experiment.

The bacteria in the sputum sample are likely to be bile soluble. This is because the presence of Gram positive lancet-shaped cocci in pairs, along with the fact that they are bile soluble, suggests that the bacteria are Streptococcus pneumoniae. Streptococcus pneumoniae is a common cause of pneumonia and is known to be bile soluble.

The correct answer is "Transient watery diarrhea." The patient's symptoms of gas in the tissues and severe pain in the injured leg are consistent with gas gangrene, which is most commonly caused by Clostridium perfringens. This bacterium is also commonly associated with transient watery diarrhea, making it the most likely microorganism responsible for the patient's condition.

The correct answer is phospholipids. Phospholipids are a major component of cell membranes and play a crucial role in maintaining the integrity and function of cells. When injected into mice, the toxic substance produced by C. perfringens causes massive hemolysis (breakdown of red blood cells) and tissue necrosis (death of cells and tissues). This suggests that the substance targets and disrupts the phospholipids in the cell membranes, leading to the observed lethal effects.

The patient's presentation with severe headache, dyspnea, and absence of breath sounds over the left middle lung fields suggests a pulmonary infection. The sputum stain revealing partially acid-fast bacilli with branching rods points towards Nocardia asteroides as the most likely cause. Nocardia asteroides is a gram-positive, partially acid-fast, branching filamentous bacterium that can cause pulmonary infections, especially in immunocompromised individuals. Mycobacterium avium-intracellulare, Mycobacterium kansasii, Mycobacterium leprae, and Mycobacterium tuberculosis are all mycobacteria but are less likely to be the cause in this case based on the clinical presentation and sputum stain results.

The most likely causal agent in this case is a marked lecithinase producer. Lecithinase is an enzyme that breaks down lecithin, a phospholipid found in cell membranes. The presence of blebs and discoloration of the skin in the affected area suggests tissue damage, which could be caused by the action of lecithinase. Additionally, the progression of symptoms from the perineal area to the leg, thigh, and buttock, along with the development of shock, indicates a spreading infection. A marked lecithinase producer would be capable of causing such severe tissue damage and systemic infection.

Staphylococcus epidermidis is the most likely cause of the woman's symptoms. The patient's history of cardiac valve replacement suggests the possibility of infective endocarditis, and the presence of a diastolic murmur further supports this diagnosis. Staphylococcus epidermidis is a common cause of infective endocarditis in patients with prosthetic heart valves. The organism is catalase-positive and coagulase-negative, which matches the characteristics of the gram-positive cocci isolated from the patient's blood cultures.

The presence of 0157:H7 strains of E. coli in the bloody stools of the children indicates an infection with this particular strain of bacteria. Hemolytic uremic syndrome (HUS) is a serious complication that can occur after an E. coli infection, especially in young children. HUS is characterized by the destruction of red blood cells, low platelet count, and kidney failure. Therefore, these young children would be at an increased risk for developing hemolytic uremic syndrome.

The correct answer suggests that the organism most likely to be isolated from the leftover beans is a gram-positive bacillus that can grow on a low oxidation-reduction medium. This means that the organism is a type of bacteria that has a thick cell wall and stains purple in the Gram stain test. It is also able to grow in an environment with low oxygen levels. These characteristics can help in identifying the organism and determining its potential role in causing the visual problems experienced by the individuals who consumed the home-canned green beans.

Pathogenic Corynebacterium diphtheriae contains an integrated temperate phage, which is not found in nonpathogenic normal flora diphtheroids. This integrated temperate phage is a type of bacteriophage that has inserted its genetic material into the bacterial genome. This phage can remain dormant within the bacterial cell, but under certain conditions, it can become active and initiate the production of toxins that contribute to the pathogenicity of C. diphtheriae. Nonpathogenic diphtheroids, on the other hand, do not possess this integrated temperate phage.

A prophage is created through site-specific recombination of a temperate phage and bacterial DNA. This process involves the integration of the temperate phage's DNA into the bacterial DNA, resulting in the prophage. The temperate phage is able to exist in a dormant state within the bacterial cell, replicating along with the host DNA. This integration occurs at specific sites in the bacterial DNA, allowing for the prophage to be stably maintained within the host genome.

Capsular swelling refers to the enlargement of the capsule surrounding the bacterium. Streptococcus pneumoniae is known to have a capsule that protects it from the host's immune system. If the capsule swells, it becomes thicker and more difficult for the bacterium to evade the immune response. Therefore, antibodies that cause capsular swelling would prevent the bacterium from causing a fatal infection in mice, as it would make it more vulnerable to the immune system's attack.

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.

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.

Cell-mediated immunity is the most important process in eliminating the bacteria from the body. This type of immunity involves the activation of T cells, which can directly kill infected cells or activate other immune cells to destroy the bacteria. In the case of the motile Gram positive rods, cell-mediated immunity would be crucial in eliminating these bacteria from the body as it can specifically target and destroy infected cells. Terminal complement cascade, eosinophil action, immunoglobulin secretion, and neutrophil oxidative burst may also play a role in the immune response, but cell-mediated immunity is the most important process in this scenario.

Urease is the most important virulence factor for the causal agent in this scenario. Urease is an enzyme that breaks down urea into ammonia and carbon dioxide. This can lead to the formation of alkaline urine, which creates an environment conducive to the growth of certain bacteria, such as Proteus mirabilis. These bacteria are known to cause urinary tract infections and can contribute to the symptoms described by the patient, including intense lower back pain and a burning sensation upon urination.

The characteristics described in the question, such as a febrile, comatose young man with a petechial rash, Kernig and Brudzinski signs, and abnormal CSF findings (high protein, low glucose, high WBC count), are consistent with meningitis. The presence of a Gram-negative, oxidase-positive diplococcus as the most likely causal agent suggests Neisseria meningitidis, which is a common cause of bacterial meningitis in young adults.

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.

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.

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.

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.

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.

The presence of gram-negative kidney bean-shaped cocci in the blood cultures suggests that the patients are infected with a gram-negative bacterium. The symptoms described, including the distinctive rash, are consistent with meningococcal meningitis, which is caused by Neisseria meningitidis. Outer membrane lipooligosaccharide (LOS) is a major virulence factor of N. meningitidis and is responsible for the symptoms and complications associated with the infection. LOS induces an inflammatory response and can cause damage to blood vessels, leading to sepsis and meningitis. Therefore, the blood concentration of outer membrane lipooligosaccharide would show the closest correlation with these patients' morbidity and mortality.

The observed phenomenon can most likely be accounted for by chromosomal fragment uptake from media. This is because when Strain 1 is injected with a lysate of Strain 2, the mice die, indicating that some genetic material from Strain 2 is being transferred to Strain 1. Chromosomal fragment uptake from media involves the uptake of DNA fragments from the surrounding environment, which can lead to genetic transfer between bacterial strains. This process would explain why the introduction of Strain 2 lysate has an effect on Strain 1.

The correct answer is "Blocks release of acetylcholine." This toxin is likely causing botulism, which is characterized by flaccid paralysis. Botulinum toxin inhibits the release of acetylcholine at the neuromuscular junction, leading to muscle weakness and paralysis. This is achieved by cleaving SNARE proteins, which are responsible for the fusion of synaptic vesicles containing acetylcholine with the presynaptic membrane. As a result, acetylcholine cannot be released into the synaptic cleft, leading to muscle weakness and paralysis.

Peptidoglycan is the major component that protects the causal agent from osmotic damage. Peptidoglycan is a structural component of the cell wall in gram-positive bacteria, which includes the causal agent in this case. It provides rigidity and strength to the cell wall, preventing it from bursting or collapsing due to changes in osmotic pressure. This is particularly important in gram-positive bacteria because they lack an outer membrane like gram-negative bacteria, making them more susceptible to osmotic damage.

The causal agent possesses a spore.

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.

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.

The most likely route of meningeal contamination in this patient is through the pharynx, blood, choroid plexus, and then the meninges. This is because the Gram stain of the cerebrospinal fluid sample showed bean-shaped Gram-negative cocci in pairs, which is indicative of Neisseria meningitidis, a common cause of bacterial meningitis. Neisseria meningitidis is typically found in the pharynx and can enter the bloodstream, allowing it to reach the meninges. The choroid plexus is a network of blood vessels in the brain that produces cerebrospinal fluid, so it is possible for the bacteria to contaminate the CSF through this route.

Subterminal spore formation refers to the production of spores by bacteria at the end of their cells. These spores are highly resistant to harsh conditions and can survive for long periods of time. In the given scenario, the bacterial product injected into the muscles of patients with dystonias produces temporary relief of symptoms. The presence of subterminal spore formation suggests that the bacteria are capable of surviving in adverse conditions within the patient's muscles, allowing them to continue producing the beneficial product.

The patient's symptoms of fever, headache, myalgias, malaise, cough, and throat pain are consistent with a respiratory infection. The progression to dyspnea, chest pain, and productive cough suggests a worsening of the infection. Staphylococcus aureus is a common cause of respiratory infections and can lead to severe pneumonia. Additionally, the fact that the patient's granddaughters had similar symptoms suggests a possible transmission of the pathogen within the family. Therefore, Staphylococcus aureus is the most likely pathogen to be isolated from the patient's sputum.

The clumping of blood in the anticoagulated tube followed by rapid uncoagulation when warmed suggests the presence of cold agglutinins, which are antibodies that can cause agglutination of red blood cells at low temperatures. Mycoplasma pneumoniae is known to be associated with cold agglutinin disease, a condition characterized by the production of these antibodies. Therefore, Mycoplasma pneumoniae is the most likely organism responsible for this patient's condition.

Epiglottitis is a bacterial infection caused by Haemophilus influenzae type b (Hib), which is known to have a capsule composed of polyribitol ribose phosphate (PRP). Antibodies against PRP provide protection against Hib infection, including epiglottitis. Therefore, the high serum level of antibodies against PRP in the 5-year-old boy suggests that he is protected against epiglottitis.

The correct answer is "Requires iron and cysteine for growth." This property is consistent with Legionella pneumophila, the organism causing Legionnaires' disease. Legionella pneumophila is an intracellular gram-negative bacterium that requires iron and cysteine for growth. It is commonly associated with water sources such as cooling towers, hot tubs, and plumbing systems, and can cause severe pneumonia in susceptible individuals, especially those with a history of smoking.

Lipopolysaccharide (LPS) is a component of the outer membrane of Gram-negative bacteria like Escherichia coli. It is a potent endotoxin that can trigger a severe immune response in the body, leading to septic shock. In this case, the patient developed sepsis due to an E. coli infection, and the subsequent development of septic shock suggests that LPS from the bacteria played a role in causing the shock. Therefore, lipopolysaccharide is the most likely structure on the bacterium responsible for causing septic shock in this patient.

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.

The correct answer is "Synthesis of an extracellular polysaccharide matrix". Coagulase-negative Staphylococcus, commonly found on the skin, can cause infections in patients with indwelling medical devices, such as ventriculoperitoneal shunts. The bacteria are able to adhere to the surface of the device and form a biofilm, which is a polysaccharide matrix that protects them from the host immune response and antibiotics. This biofilm formation is a key virulence mechanism that allows the bacteria to persist and cause disease in these patients.

The correct answer is colonization of the hospital water system. This is because the microorganism isolated in the cases is visualized with silver stains and demonstrates slow growth on complex media, indicating that it is likely a waterborne pathogen. The increased incidence of nosocomial pneumonias in patients with long hospitalizations suggests that they are being exposed to the pathogen over an extended period of time, possibly through contaminated water sources within the hospital.

The patient's presentation with fever, malaise, intense pain over the right thigh, and local warmth suggests an acute infection. The fact that she has a long history of painful crises indicates a chronic condition, most likely sickle cell disease. Patients with sickle cell disease are at increased risk for infections, particularly by encapsulated organisms such as Salmonella. Therefore, Salmonella is the most likely organism causing her clinical presentation.

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.

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.

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.

Vaccination of young adults would be the most appropriate prevention measure to control the rise in neonatal tetanus in a developing country. This is because neonatal tetanus is often caused by unhygienic practices during childbirth, such as cutting the umbilical cord with unsterile instruments. By vaccinating young adults, who are often the ones delivering the babies, they can be protected against tetanus and reduce the risk of transmitting the infection to newborns. This measure helps to break the cycle of transmission and prevent neonatal tetanus cases from occurring.

The observed bacterial growth pattern of mycobacteria growing in parallel chains, also known as "serpentine cords," is most strongly correlated with virulence. Virulence refers to the ability of a pathogen to cause disease in a host. The formation of serpentine cords by mycobacteria is a characteristic feature of highly virulent strains, particularly Mycobacterium tuberculosis. This growth pattern allows the bacteria to evade immune defenses and establish a persistent infection within the host. Therefore, the presence of serpentine cords in sputum cultures suggests a higher likelihood of a virulent mycobacterial infection.

Intrapartum ampicillin is the most effective measure to prevent the condition described in the question. The patient in the question is presenting with symptoms of early-onset neonatal sepsis, which is commonly caused by Group B Streptococcus (GBS) infection. Intrapartum ampicillin is the recommended treatment for GBS colonization in pregnant women to prevent transmission of the infection to the newborn. Penicillin at 30 weeks gestation is not effective as it is given too early in the pregnancy. Maternal vaccination may help prevent certain infections, but it is not specifically targeted towards GBS. Postnatal immunoglobulin and breastfeeding restriction do not directly address the prevention of GBS infection during childbirth.

The patient's symptoms of sudden-onset fever, chills, productive cough, and left-sided chest pain that worsens with deep breathing, along with bronchial breath sounds over the left lower lung, are indicative of pneumonia. Streptococcus pneumoniae is the most common cause of community-acquired pneumonia, especially in patients with HIV. Additionally, the patient's recent CD4+ lymphocyte count of 800 cells/μL suggests that his immune system is compromised, making him more susceptible to infections, including pneumonia caused by Streptococcus pneumoniae.

The correct answer is ADP ribosylation of Gi. ADP ribosylation is a process in which ADP-ribose is transferred from NAD+ to a target protein, modifying its function. In this case, the toxin involved in the patient's condition is causing ADP ribosylation of the G protein subunit Gi. This modification disrupts the normal function of Gi, which is involved in inhibiting the production of cyclic AMP (cAMP). As a result, cAMP levels increase, leading to excessive fluid secretion and inflammation in the respiratory tract, causing the symptoms observed in the patient.

The correct answer describes the causal agent as a gram-negative curved rod that is microaerophilic. This is likely referring to Helicobacter pylori, a bacterium known to cause gastric ulcers and gastritis. H. pylori has a characteristic curved shape and is able to survive in environments with low oxygen levels. The positive urease breath test is a diagnostic test used to detect the presence of H. pylori in the stomach.

Pseudomonas aeruginosa is the most likely causal agent in this case because it is known to cause necrotizing soft tissue infections, especially in immunocompromised individuals like this patient who is positive for HIV. The black center and erythematous margin of the lesion are consistent with the characteristic appearance of a necrotic lesion caused by Pseudomonas aeruginosa. Bacillus anthracis, Clostridium perfringens, Enterococcus faecalis, and Staphylococcus aureus are not typically associated with this presentation.

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.