USMLE Step 1 Question (Microbiology)
Make a diagnosis based on the organism's characteristics
The following is a high-yield USMLE Step 1 microbiology question.
It is followed by a deep dive into each answer choice—the answer can be found at the bottom of the post.
Good luck!
QUESTION:
A 5-year-old male is being evaluated by his primary care physician for a two-day history of sore throat, fever, and fatigue. The patient’s temperature is 102.1°F, heart rate is 100/minute, respiratory rate is 18/minute, and blood pressure is 100/70 mmHg. Physical examination reveals a whitish lesion on the posterior pharynx that bleeds when scraped. The patient has not been vaccinated due to his parent’s personal beliefs. Which of the following best describes the causative organism?
A. Gram-positive clusters, coagulase-negative
B. Gram-positive rods, angular arrangement
C. Anaerobic branching filaments
D. Gram-positive, beta-hemolytic chains
E. Gram-positive, alpha-hemolytic chains
To see the answer, scroll to the bottom of the post. If you’re unsure, here’s a DEEP-DIVE into each answer choice.
DEEP-DIVE:
The description of the patient's symptoms, particularly the whitish lesion on the posterior pharynx that bleeds when scraped and the lack of vaccination, suggests a vaccine-preventable condition.
A. Gram-positive clusters, coagulase-negative
This description is characteristic of Staph epidermidis and Staph saprophyticus.
Clinical Context: Due to its ability to form biofilm, S. epidermidis is a leading cause of infections associated with medical devices such as catheters, prosthetic joints, and heart valves. Staph saprophyticus is the second most common cause of uncomplicated UTI, especially in young women (E. coli is the #1 cause).
Remember: A key virulence factor of S. epidermidis is its ability to produce polysaccharide intracellular adhesin (PIA), which facilitates biofilm formation on medical device surfaces. These complex communities of bacteria are encased in a protective matrix that adheres to surfaces, making infections difficult to treat.
B. Gram-positive rods, angular arrangement
This is characteristic of Corynebacterium diphtheriae, the causative agent of diphtheria. Diphtheria can cause pharyngitis with a characteristic greyish or whitish pseudomembrane on the tonsils, pharynx, or nose, which can bleed upon scraping. The patient's lack of vaccination increases the risk for diphtheria.
Clinical Context: Not only can diphtheria lead to the formation of a pseudomembrane in the throat and difficulty breathing, but it may also lead to potentially fatal heart and neurological effects.
Remember: The toxin produced by certain strains of C. diphtheriae, mediated by a bacteriophage, inhibits protein synthesis in host cells, leading to cell death and the characteristic pseudomembrane formation.
C. Anaerobic branching filaments
This characteristic is typical of Actinomyces spp., which are more commonly associated with oral and cervicofacial infections that form abscesses or fistulas rather than acute pharyngitis.
Clinical Context: Actinomyces are known for causing chronic conditions like actinomycosis, which can present as cervicofacial, thoracic, or abdominal infections, often with abscess formation and draining sinuses.
These organisms are part of the normal flora of the mouth and respiratory tract. Infection usually follows disruption of the mucosal barrier, allowing the bacteria to invade deeper tissues.
D. Gram-positive, beta-hemolytic chains
These features are characteristic of Streptococcus species, particularly Group A Streptococcus (Streptococcus pyogenes) and Group B Streptococcus, which are known for their beta-hemolytic activity.
Clinical Context: Strep pyogenes is responsible for many clinical conditions, ranging from mild to life-threatening. Some of the most commonly tested S. pyogenes-related infections include pharyngitis, impetigo, Scarlet fever, necrotizing fasciitis, and rheumatic fever.
Remember: Beta-hemolysis refers to the complete lysis of red blood cells near the bacterial colonies on blood agar; this property can be used to differentiate bacterial species.
E. Gram-positive, alpha-hemolytic chains
This description is characteristic of viridans streptococci and Strep pneumonia.
Clinical Context: Streptococcus pneumoniae is a significant cause of pneumonia, meningitis, otitis media, and sinusitis, especially in children and the elderly. Viridans group strep (VGS) is commonly linked to endocarditis and dental caries.
Remember: S. mutans plays a pivotal role in developing dental caries through its ability to adhere to tooth surfaces. It forms biofilms known as dental plaque and ferments dietary carbohydrates (mainly sucrose). The fermentation process produces acid, causing demineralization of tooth enamel—the primary mechanism behind the formation of dental caries.
Given the patient’s presentation, vaccination status, and presence of a gram-positive rod with an angular arrangement, Corynebacterium diphtheriae is the most likely causative agent. Diphtheria presents with a sore throat, fever, and a characteristic pseudomembrane in the throat, which can bleed if scraped.
FINAL ANSWER: B. Gram-positive rods, angular arrangement
