USMLE Step 1 Quiz
Practice Microbial Laboratory Identification in USMLE Step 1 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
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A 28-year-old woman returns from a mission trip to Nigeria and develops high fevers that occur every 48 hours, accompanied by chills and profuse sweating. Her physician suspects malaria. A sample of her peripheral blood is sent to the laboratory for microscopic examination.
Which stain is the standard method for visualizing parasites, such as Plasmodium species, within red blood cells on a blood smear?
This quiz focuses on Microbial Laboratory Identification, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.
Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.
A 28-year-old woman returns from a mission trip to Nigeria and develops high fevers that occur every 48 hours, accompanied by chills and profuse sweating. Her physician suspects malaria. A sample of her peripheral blood is sent to the laboratory for microscopic examination.
Which stain is the standard method for visualizing parasites, such as Plasmodium species, within red blood cells on a blood smear?
Explanation: The diagnosis of malaria is typically made by microscopic examination of a peripheral blood smear. The Giemsa stain is the preferred method for this purpose. It differentially stains the parasites' nuclei red and cytoplasm blue, allowing for visualization of the intraerythrocytic ring forms, trophozoites, schizonts, and gametocytes of Plasmodium species and enabling species identification based on morphology.
A 22-year-old man presents with a 2-day history of purulent urethral discharge and dysuria after unprotected sexual contact. A urethral swab is sent for culture. To isolate the fastidious gram-negative diplococcus responsible for his condition, the sample is plated on a selective medium containing vancomycin, colistin, nystatin, and trimethoprim.
What is the name of this selective medium?
Explanation: The clinical presentation is classic for gonorrhea, caused by Neisseria gonorrhoeae. This organism is fastidious and is often found in sites with normal flora. Thayer-Martin agar (or modified Thayer-Martin) is a selective medium used for its isolation. It is a chocolate agar base enriched with antibiotics: vancomycin to inhibit gram-positive bacteria, colistin to inhibit other gram-negative bacteria, nystatin to inhibit fungi, and trimethoprim to inhibit swarming Proteus.
A 50-year-old immigrant from Southeast Asia with a chronic cough undergoes a bronchoscopy with bronchoalveolar lavage. The physician suspects tuberculosis. The sample is sent to the microbiology lab for culture. The lab technician inoculates the sample onto a specific egg-based medium that contains malachite green to inhibit other bacteria.
Given the slow-growing nature of the suspected organism, which medium will be incubated for up to 8 weeks to allow for growth?
Explanation: Mycobacterium tuberculosis is a very slow-growing organism, often taking 3-6 weeks to form visible colonies. The classic medium for its culture is Lowenstein-Jensen (LJ) medium. LJ medium is an egg-based medium containing nutrients required for mycobacterial growth, as well as malachite green to suppress the growth of contaminating gram-positive bacteria from the specimen.
An 8-year-old boy presents with a severe sore throat, fever of 39.4°C (103°F), and tonsillar exudates. A rapid antigen detection test for Streptococcus pyogenes is positive. A throat culture is performed on blood agar and shows small colonies surrounded by a clear zone of complete hemolysis.
This zone of complete hemolysis is known as beta-hemolysis. Which test is used to presumptively identify Streptococcus pyogenes from other beta-hemolytic streptococci?
Explanation: Streptococcus pyogenes (Group A Streptococcus) is a common cause of pharyngitis and is beta-hemolytic. To differentiate it from other beta-hemolytic streptococci, such as Streptococcus agalactiae (Group B), the bacitracin sensitivity test is used. S. pyogenes is sensitive to bacitracin (shows a zone of inhibition), whereas other beta-hemolytic streptococci are typically resistant.
A hospital blood bank screens all donated blood units for hepatitis C virus (HCV) infection. The primary screening test is a highly sensitive assay that detects antibodies against HCV proteins. This test involves immobilizing viral antigens on a solid phase, adding patient serum, and then using a secondary antibody linked to a color-generating enzyme to detect bound patient antibodies.
What is the name of this widely used serologic technique?
Explanation: The technique described is the enzyme-linked immunosorbent assay (ELISA), specifically an indirect ELISA for detecting antibodies. It is a common, highly sensitive method used for screening for various infectious diseases. The key components are an antigen-coated solid phase, the patient's serum (containing primary antibodies), and an enzyme-conjugated secondary antibody that detects the primary antibody, leading to a color change upon addition of a substrate.
A 30-year-old intravenous drug user presents with a painful, fluctuant abscess on his forearm. Aspiration of the abscess yields purulent material. Gram stain shows gram-positive cocci in grape-like clusters. The isolate grows on blood agar and is tested in the laboratory.
Which of the following sets of results would be most consistent with Staphylococcus aureus?
Explanation: Gram-positive cocci in clusters are characteristic of Staphylococcus species. The first step in differentiation is the catalase test, which separates catalase-positive staphylococci from catalase-negative streptococci. Staphylococcus aureus is distinguished from other staphylococci (e.g., S. epidermidis, S. saprophyticus) by the coagulase test. S. aureus is coagulase-positive, meaning it produces an enzyme that clots plasma.
A 24-year-old woman presents with dysuria and urinary frequency. A clean-catch urine sample is sent for culture. The laboratory reports growth of over 100,000 colony-forming units/mL of a gram-negative rod. The organism was plated on MacConkey agar, where it produced vibrant pink colonies.
The formation of pink colonies on this medium indicates the organism is capable of which of the following biochemical reactions?
Explanation: MacConkey agar is a selective and differential medium used to isolate gram-negative enteric bacteria. It contains bile salts to inhibit gram-positive organisms and lactose as a fermentable carbohydrate. Organisms that ferment lactose produce acid, which lowers the pH and causes the neutral red indicator to turn pink or red. Non-lactose fermenters form colorless colonies.
A 45-year-old man who is homeless and lives in a shelter presents with a 3-month history of a persistent cough, night sweats, weight loss, and hemoptysis. A sputum sample is obtained for microscopic examination to identify the suspected etiologic agent.
Which of the following staining techniques is most appropriate for visualizing this pathogen?
Explanation: The patient's clinical presentation is classic for pulmonary tuberculosis, caused by Mycobacterium tuberculosis. Mycobacteria have a cell wall with a high mycolic acid content, which prevents them from being stained by the Gram stain. The Ziehl-Neelsen (or acid-fast) stain is used to identify these organisms. They resist decolorization by acid-alcohol and appear red against a blue background.
A 2-year-old unvaccinated child is brought to the emergency department with fever, nuchal rigidity, and lethargy. A lumbar puncture is performed, and cerebrospinal fluid (CSF) is sent for culture. The culture grows small, pleomorphic gram-negative rods only on chocolate agar, but not on standard blood agar.
The growth requirement for chocolate agar suggests the organism requires which of the following factors?
Explanation: The scenario describes an infection likely caused by Haemophilus influenzae. This organism is fastidious and requires both factor V (NAD+) and factor X (hematin) for growth. Standard blood agar contains factor X but lacks sufficient free factor V, which is released when red blood cells are lysed by heat, as in the preparation of chocolate agar. Therefore, H. influenzae grows on chocolate agar but not on blood agar.
A 55-year-old patient in the intensive care unit with severe burns develops a wound infection characterized by blue-green purulent discharge and a grape-like odor. A culture of the wound grows a gram-negative rod that is a non-lactose fermenter.
Which of the following biochemical test results is most expected for this organism?
Explanation: The clinical features (burn patient, blue-green pigment from pyocyanin, fruity odor) are classic for Pseudomonas aeruginosa. This organism is a gram-negative, non-lactose-fermenting rod. A key biochemical characteristic that helps differentiate Pseudomonas from many members of the Enterobacteriaceae family is the oxidase test. Pseudomonas aeruginosa is oxidase-positive.
A 34-year-old man with AIDS (CD4 count of 45 cells/mm³) presents with a severe headache, fever, and neck stiffness. A lumbar puncture is performed. Cerebrospinal fluid analysis reveals an elevated opening pressure. A wet mount of the CSF is prepared for microscopic examination.
Which of the following staining methods would be most useful for visualizing the prominent polysaccharide capsule of the suspected pathogen?
Explanation: The presentation of meningitis in a severely immunocompromised patient suggests an opportunistic infection, most commonly Cryptococcus neoformans. This is an encapsulated yeast. The India ink stain is a negative staining technique where the ink particles are excluded by the thick polysaccharide capsule, creating a clear halo around the yeast cell against a dark background, making it easy to visualize.
An 80-year-old male nursing home resident with an indwelling urinary catheter develops a urinary tract infection. Urine culture grows a motile, gram-negative rod that produces colorless colonies on MacConkey agar. A key feature of this organism is its ability to produce large amounts of an enzyme that splits urea into ammonia and carbon dioxide, leading to an alkaline urine pH and struvite stone formation.
The activity of which of the following enzymes is responsible for this organism's characteristic effect on urine?
Explanation: The scenario describes a urinary tract infection likely caused by Proteus mirabilis, which is notorious for causing UTIs in catheterized patients and forming struvite (magnesium ammonium phosphate) stones. This is due to its production of a potent urease enzyme, which hydrolyzes urea to ammonia. The ammonia raises the urine pH, promoting the precipitation of minerals and formation of stones. This strong urease activity is a key diagnostic feature.
A 3-day-old neonate develops fever, poor feeding, and respiratory distress. Blood cultures are drawn and grow a gram-positive coccus that forms chains and demonstrates a narrow zone of beta-hemolysis on blood agar. The organism is catalase-negative.
Which laboratory test, involving co-culturing with Staphylococcus aureus, would produce an arrowhead-shaped zone of enhanced hemolysis and confirm the identity of this organism?
Explanation: The clinical scenario of neonatal sepsis with a gram-positive, catalase-negative, beta-hemolytic coccus is highly suggestive of Streptococcus agalactiae (Group B Streptococcus). The CAMP test is used for its identification. S. agalactiae produces a diffusible protein (CAMP factor) that acts synergistically with the beta-hemolysin of Staphylococcus aureus to create an enhanced, arrowhead-shaped zone of hemolysis where the two organisms are streaked perpendicularly on a blood agar plate.
A 20-year-old college student returns from a trip to Mexico with profuse, watery diarrhea. A stool sample is cultured on eosin-methylene blue (EMB) agar. The plate shows growth of colonies that exhibit a characteristic metallic green sheen.
This distinctive appearance on EMB agar is a classic finding for which of the following organisms?
Explanation: Eosin-methylene blue (EMB) agar is a selective and differential medium for gram-negative rods. The dyes inhibit gram-positive bacteria. It differentiates based on lactose fermentation. Escherichia coli is a vigorous fermenter of lactose (and sucrose), producing a large amount of acid that causes the dyes to precipitate on the colony surface, resulting in a characteristic metallic green sheen. This finding is highly specific for E. coli.
An elderly patient with multiple comorbidities is diagnosed with pneumonia. A sputum culture grows gram-positive cocci that are alpha-hemolytic and optochin-sensitive. For epidemiological tracking, a serotyping method is used where polyvalent antisera are mixed with a suspension of the bacteria.
Observation of microscopic capsular swelling in this procedure confirms the identity of Streptococcus pneumoniae. What is this serological reaction called?
Explanation: The Quellung reaction (from the German word for 'swelling') is a biochemical reaction in which antibodies bind to the bacterial capsule of certain encapsulated bacteria, such as Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis. This antibody binding causes the capsule to swell and become more refractile, making it visible under a microscope. It is used for serotyping these organisms.
A 68-year-old man with a prosthetic heart valve undergoes a colonoscopy. Two weeks later, he develops fever, chills, and a new heart murmur. Blood cultures are drawn and grow a gram-positive coccus in chains that is catalase-negative and gamma-hemolytic. The organism grows in a broth containing 6.5% NaCl.
Growth of this organism on which of the following media would also be expected?
Explanation: The patient's presentation suggests subacute bacterial endocarditis. The source is likely the gastrointestinal tract (colonoscopy). The microbiological findings—gram-positive cocci, catalase-negative, and growth in 6.5% NaCl—are characteristic of Enterococcus faecalis. Enterococci are distinguished by their ability to grow in harsh conditions, including high salt and the ability to hydrolyze esculin in the presence of bile. This is tested on bile esculin agar, where a positive result (blackening of the agar) is seen.
A 72-year-old heavy smoker who recently stayed at a hotel for a conference presents with fever, myalgia, headache, and a non-productive cough. Chest X-ray shows a patchy infiltrate. Sputum Gram stain shows many neutrophils but few organisms. A special culture medium buffered to a pH of 6.9 and supplemented with L-cysteine and iron is required for isolation.
The described medium is most likely which of the following?
Explanation: The clinical scenario is classic for Legionnaires' disease, caused by Legionella pneumophila. This organism is a fastidious, faintly-staining gram-negative rod that does not grow on standard media. It requires specific supplementation with L-cysteine and iron for growth. The medium of choice for isolating Legionella is buffered charcoal yeast extract (BCYE) agar.
A 5-year-old unimmunized child, recently immigrated from Eastern Europe, presents with a severe sore throat, fever, and malaise. Physical examination reveals a thick, gray, adherent pseudomembrane over the tonsils and pharynx. The physician suspects diphtheria.
To confirm the diagnosis, a culture is performed on a special medium that contains potassium tellurite, which inhibits most upper respiratory tract flora and is reduced by the suspected organism to produce black colonies. What is this medium?
Explanation: The clinical picture is pathognomonic for diphtheria, caused by Corynebacterium diphtheriae. For its isolation from the respiratory tract, selective media are required. Cysteine-tellurite blood agar is a commonly used medium. C. diphtheriae can reduce the potassium tellurite in the medium to metallic tellurium, resulting in the formation of characteristic gray or black colonies, aiding in its identification.
A 29-year-old woman presents with a urinary tract infection. Urine culture grows a gram-negative rod that is a lactose fermenter on MacConkey agar. The laboratory performs a biochemical test by inoculating the isolate into a medium containing tryptophan. After incubation, Kovac's reagent is added, and a red color develops in the upper layer of the broth.
This positive result indicates the presence of which enzyme?
Explanation: The test described is the indole test. It detects the production of the enzyme tryptophanase, which hydrolyzes the amino acid tryptophan into indole, pyruvic acid, and ammonia. The addition of Kovac's reagent (containing p-dimethylaminobenzaldehyde) detects the indole, producing a red-violet color. This test is crucial for differentiating members of the Enterobacteriaceae; for instance, E. coli is indole-positive, while Klebsiella and Enterobacter are typically indole-negative.
A 23-year-old sexually active woman is diagnosed with pelvic inflammatory disease. Her endocervical swab is sent for testing for the two most common causative organisms. While one organism can be cultured on Thayer-Martin medium, the other is an obligate intracellular bacterium that cannot be grown on artificial media.
What is the most sensitive and specific diagnostic method for detecting this obligate intracellular pathogen?
Explanation: The scenario refers to Chlamydia trachomatis, a common cause of PID and an obligate intracellular bacterium. Because it cannot be cultured on standard bacteriological media, traditional diagnostic methods are insensitive. Nucleic acid amplification tests (NAATs), such as PCR, are the diagnostic gold standard. They detect specific DNA or RNA sequences of the organism, offering very high sensitivity and specificity from clinical samples like cervical swabs or urine.