USMLE PRACTICE QUESTION #85
TOPIC: Infectious disease / microbiology.
Today’s question will test your understanding of a commonly tested infectious disease.
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A 28-year-old male presents to the emergency department with a high fever, severe headache, and muscle aches. He reports that he has been feeling unwell for the past week. He recently returned from a trip to Southeast Asia where he participated in outdoor adventure activities, including swimming in rivers and hiking through jungles. His vitals include temperature of 101.9°F, blood pressure 130/90 mmHg, pulse 100/minute, and respirations 16/minute. On examination, he appears jaundiced, and there is conjunctival suffusion. Laboratory results reveal elevated liver enzymes, thrombocytopenia, and acute kidney injury. Blood and urine cultures have been sent for further testing. Which of the following is the most likely underlying mechanism behind the cause of this patient's infection?
A) Direct invasion of tissue by an intracellular parasite
B) Bacterial endotoxin release resulting in systemic inflammation
C) Binding of a bacterial exotoxin to host cell receptors
D) Invasion of the bloodstream by a spirochete bacterium
E) Disruption of the mucosal barrier by a fungal organism
F) Viral-induced cytopathic effect on host cells
G) Formation of biofilm by a bacterial pathogen
Detailed Breakdown of Answers + Correct Answer Below ⏬
ANSWER + QUESTION BREAKDOWN
The MENTAL MODEL used to answer this question comes from our detailed test-taking skills masterclass (check it out… it’s like putting your test-taking skills on steroids!) 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 is the most likely underlying mechanism behind the cause of this patient's infection?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order. 1st: Diagnose the condition, and 2nd: Identify the causative organism’s underlying mechanism of infection.
Step 3: Read the vignette carefully and ask yourself: “Based on my diagnosis, how did the causative organism infect the patient?”
Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.
GENERAL ANALYSIS
The 28-year-old male presents with a high fever, severe headache, muscle aches, jaundice, conjunctival suffusion, elevated liver enzymes, thrombocytopenia, and acute kidney injury after recent travel to Southeast Asia, where he engaged in outdoor activities like swimming in rivers. These findings suggest leptospirosis, a zoonotic bacterial infection caused by the spirochete Leptospira.
ANSWER CHOICES:
CHOICE A: Direct invasion of tissue by an intracellular parasite
Explanation: This describes infections like malaria or toxoplasmosis. Leptospirosis is caused by a spirochete bacterium, not an intracellular parasite.
CHOICE B: Bacterial endotoxin release resulting in systemic inflammation
Explanation: This mechanism is typical of Gram-negative bacterial infections and sepsis. While leptospirosis can cause systemic inflammation, it is unrelated to this mechanism.
CHOICE C: Binding of a bacterial exotoxin to host cell receptors
Explanation: Exotoxins are produced by bacteria like Clostridium or Staphylococcus aureus. Leptospira does not cause disease through exotoxin production.
CHOICE D: Invasion of the bloodstream by a spirochete bacterium
Explanation: When exposed to contaminated water, Leptospira species can enter the body through cuts or abrasions in the skin or mucous membranes. The spirochetes invade the bloodstream and disseminate to various organs, leading to systemic inflammation and damage to the liver and kidneys.
CHOICE E: Disruption of the mucosal barrier by a fungal organism
Explanation: This describes fungal infections such as candidiasis. The patient’s symptoms aren’t consistent with a fungal infection.
CHOICE F: Viral-induced cytopathic effect on host cells
Explanation: This describes viral infections that cause cell damage directly.
CHOICE G: Formation of biofilm by a bacterial pathogen
Explanation: Biofilm formation is seen in chronic infections caused by bacteria like Pseudomonas or Staphylococcus epidermidis—not leptospira.
FINAL VERDICT…
The most likely underlying mechanism behind this patient's condition is invasion of the bloodstream by the spirochete bacterium Leptospira, which causes leptospirosis. This zoonotic infection is commonly contracted through exposure to contaminated water during outdoor activities in tropical regions. The clinical presentation of fever, muscle aches, jaundice, and kidney injury is characteristic of severe leptospirosis (Weil’s disease), which often involves multi-organ dysfunction.
CORRECT ANSWER: D) Invasion of the bloodstream by a spirochete bacterium
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That’s it for question 85!
See ya tomorrow 👋