USMLE Step 1 Quiz: Microbial Laboratory Identification

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?