USMLE PRACTICE QUESTION #125
TOPIC: Cardiology/Pathology
USMLE practice question #125 will test your understanding of a commonly tested cardiovascular condition.
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A 61-year-old man with a history of mitral valve prolapse presents with low-grade fevers, night sweats, and unintentional weight loss over the last month. Physical examination reveals a blowing holosystolic murmur at the apex radiating to the axilla, splinter hemorrhages under the nails, and non-tender, erythematous lesions on the palms. Blood cultures are positive for Gram-positive, alpha-hemolytic cocci that are optochin-resistant. Which of the following additional findings would most likely be present in this patient and is associated with this pathogen?
A) Antibodies against M protein in serum
B) Bile solubility indicating capsule degradation
C) Growth in 6.5% NaCl medium
D) Production of dextrans from glucose
E) Positive coagulase test
Detailed Breakdown of Answers + Correct Answer Below ⏬
ANSWER + QUESTION BREAKDOWN
It’s important to adopt the correct MENTAL MODEL when answering USMLE questions; it saves time and increases accuracy. The mental model outlined below is a foundational component of our test-taking skills masterclass (check it out if you want to elevate your skills). Here’s how to think through this question:
Step 1. Read the last line to get to the heart of the question: “Which of the following additional findings would most likely be present in this patient and is associated with this pathogen?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order. 1st: Diagnose the condition; 2nd: Identify the correct additional finding based on your diagnosis.
Step 3: Read the vignette carefully and ask yourself: “Based on my diagnosis, the most likely addition finding is _____________________.”
Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.
GENERAL ANALYSIS
This patient’s presentation is consistent with subacute infective endocarditis, which often manifests with slower onset of fevers, weight loss, and night sweats. The non-tender, erythematous lesions on palms are Janeway lesions, and splinter hemorrhages under the nails further point to endocarditis. The organism is described as a Gram-positive, alpha-hemolytic cocci that is resistant to optochin, which is characteristic of Streptococcus viridans.
ANSWER CHOICES:
CHOICE A: Antibodies against M protein in serum
Explanation: Antibodies against M protein are specific to Streptococcus pyogenes (Group A Streptococcus) and are associated with post-infectious complications such as rheumatic fever.
CHOICE B: Bile solubility indicating capsule degradation
Explanation: Bile solubility is a feature of Streptococcus pneumoniae, which is optochin-sensitive and causes diseases like pneumonia, meningitis, and sepsis.
CHOICE C: Growth in 6.5% NaCl medium
Explanation: Growth in 6.5% NaCl is a characteristic of enterococci (e.g., Enterococcus faecalis), which can also cause endocarditis but are not alpha-hemolytic or optochin-resistant.
CHOICE D: Production of dextrans from glucose
Explanation: Viridans streptococci, including S. sanguinis and S. mitis, produce extracellular polysaccharides (dextrans) from glucose. These dextrans facilitate bacterial adherence to fibrin-platelet aggregates on damaged heart valves, a critical step in the development of infective endocarditis.
CHOICE E: Positive coagulase test
Explanation: A positive coagulase test is specific for Staphylococcus aureus, which can cause acute bacterial endocarditis.
FINAL VERDICT…
CORRECT ANSWER: D) Production of dextrans from glucose
Viridans streptococci produce dextrans from glucose, enabling them to adhere to fibrin-platelet aggregates on damaged heart valves. This mechanism plays a central role in the pathogenesis of subacute bacterial endocarditis, particularly in patients with predisposing conditions like mitral valve prolapse.
KEY CONCEPTS:
Viridans Streptococci Characteristics:
Gram-positive, catalase-negative cocci.
Alpha-hemolytic on blood agar.
Optochin-resistant and bile-insoluble.
Pathogenesis of Infective Endocarditis:
Viridans streptococci produce dextrans from glucose.
Dextrans mediate adherence to fibrin-platelet aggregates on damaged heart valves.
Commonly associated with subacute bacterial endocarditis in patients with underlying valvular abnormalities (e.g., mitral valve prolapse).
Clinical Features of Subacute Bacterial Endocarditis:
Low-grade fever, night sweats, weight loss.
Peripheral stigmata such as splinter hemorrhages, Janeway lesions, Osler nodes.
New or changing heart murmur.
Laboratory Identification:
Alpha-hemolytic Gram-positive cocci that are optochin-resistant indicate viridans streptococci.
Blood cultures confirm the diagnosis.
Differential Diagnosis for Alpha-Hemolytic Cocci:
Streptococcus pneumoniae: Optochin-sensitive, bile-soluble.
Viridans streptococci (S. sanguinis, S. mitis): Optochin-resistant, bile-insoluble.
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