USMLE PRACTICE QUESTION #105

TOPIC: Infectious Disease

Dec 11, 2024

In USMLE practice question #105, we’ll challenge our understanding of a super high-yield, commonly tested infectious disease topic.

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A 32-year-old woman presents to the emergency department with fever, headache, neck stiffness, and photophobia. A lumbar puncture is performed, and cerebrospinal fluid (CSF) analysis reveals the following:

•Opening pressure: 210 mmH₂O
•White blood cells: 2500 cells/mm³ (85% neutrophils)
•Glucose: 30 mg/dL (N: 50-75 mg/dL)
•Protein: 200 mg/dL (N: 15-45 mg/dL)

Which of the following histological findings is most likely associated with the causative organism for this patient’s presentation?

A) Encapsulated yeast with narrow-based budding visualized with India ink staining
B) Gram-positive diplococci in lancet shape on Gram stain
C) Perivascular lymphocytic infiltration with no organisms visible on routine staining D) Acid-fast bacilli with caseating granulomas in CNS tissue
E) Spherules containing endospores in infected tissue

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 histological findings is most likely associated with the causative organism for this patient’s presentation?”

Step 2: Is this a first-, second-, or third-order question?
Answer: 3rd order. 1st: Diagnose the condition; 2nd: Identify the most likely organism based on presentation; 3rd: Identify the causative organism’s histology

Step 3: Read the vignette carefully and ask yourself: “Based on my diagnosis and the causative organism, what histological features do I expect?”

Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.

GENERAL ANALYSIS

This vignette describes a 32-year-old woman presenting with fever, headache, neck stiffness, and photophobia, which are classic signs of meningitis. The CSF findings are pointing to a diagnosis of acute bacterial meningitis.

ANSWER CHOICES:

CHOICE A: Encapsulated yeast with narrow-based budding visualized with India ink staining

Explanation: This describes Cryptococcus neoformans, a fungal pathogen that causes cryptococcal meningitis, particularly in immunocompromised individuals such as those with HIV. Cryptococcal meningitis typically presents with lymphocytic pleocytosis, elevated opening pressure, low glucose, and high protein.

CHOICE B: Gram-positive diplococci in lancet shape on Gram stain

Explanation: This describes Streptococcus pneumoniae, a common cause of bacterial meningitis in adults. Pneumococcal meningitis typically presents with fever, headache, neck stiffness, and photophobia. This patient’s CSF findings—neutrophilic pleocytosis (85% neutrophils), low glucose (30 mg/dL), and elevated protein (200 mg/dL)—are classic for bacterial meningitis caused by Streptococcus pneumoniae.

CHOICE C: Perivascular lymphocytic infiltration with no organisms visible on routine staining

Explanation: This finding is characteristic of viral (aseptic) meningitis or certain chronic infections like viral encephalitis. Viral meningitis typically presents with lymphocytic pleocytosis in the CSF. This patient’s CSF shows neutrophilic pleocytosis and low glucose, which are more consistent with bacterial rather than viral meningitis.

CHOICE D: Acid-fast bacilli with caseating granulomas in CNS tissue

Explanation: This finding is characteristic of tuberculous meningitis caused by Mycobacterium tuberculosis. Tuberculous meningitis typically presents with lymphocytic pleocytosis, very low glucose levels, and elevated protein.

CHOICE E: Spherules containing endospores in infected tissue

Explanation: This finding is characteristic of coccidioidal infections caused by Coccidioides immitis, which can cause coccidioidal meningitis. This type of fungal infection is more common in endemic areas like the southwestern United States.

FINAL VERDICT…

The patient’s presentation—fever, headache, neck stiffness, photophobia—and CSF findings (elevated opening pressure, neutrophilic pleocytosis, low glucose, high protein) are highly suggestive of acute bacterial meningitis, most likely caused by Streptococcus pneumoniae. This bacterium is a common cause of bacterial meningitis in adults and is often seen as gram-positive diplococci in a lancet shape on Gram stain.

Thus, the most likely histological finding associated with this patient's presentation would be gram-positive diplococci in lancet shape on Gram stain.

CORRECT ANSWER: B) Gram-positive diplococci in lancet shape on Gram stain