USMLE PRACTICE QUESTION #112
TOPIC: Respiratory Histology
In USMLE practice question #112, we’ll test our ability to diagnose a commonly tested condition based on recognition of the correct gram stain finding.
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A 52-year-old man presents to the emergency department with a 3-day history of fever, productive cough, and chest pain that worsens with deep breathing. He reports coughing up thick, blood-tinged sputum. He has a history of chronic alcohol use disorder and frequently experiences episodes of aspiration. On physical examination, he appears febrile and tachypneic, with crackles and dullness to percussion over the right upper lung field. Laboratory studies reveal:
WBC count: 18,500/µL
Neutrophils: 85%
A sputum sample is sent for Gram stain and culture. Which of the following correctly describes the most likely causative organism?
A) Gram-positive cocci in chains
B) Gram-positive cocci in clusters
C) Gram-negative diplococci
D) Gram-negative rods with a thick capsule
E) Gram-negative curved rods
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 correctly describes the most likely causative organism?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 2n order. 1st: Diagnose the condition; 2nd: Identify the organism based on its gram stain characteristics.
Step 3: Read the vignette carefully and ask yourself: “Based on my suspected diagnosis, what gram stain characteristics 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 52-year-old man who presents with a 3-day history of fever, productive cough, chest pain, and blood-tinged (currant jelly) sputum. He has a history of chronic alcohol use disorder and frequent episodes of aspiration. Physical examination reveals fever, tachypnea, and crackles with dullness to percussion over the right upper lung field. Laboratory studies show an elevated white blood cell count with neutrophilia, and a chest X-ray shows consolidation in the right upper lobe.
ANSWER CHOICES:
CHOICE A: Gram-positive cocci in chains
Explanation: This describes the appearance of Streptococcus pyogenes or other streptococci. Streptococcus pneumoniae is also a common cause of pneumonia, but it typically produces rust-colored sputum, not currant jelly sputum.
CHOICE B: Gram-positive cocci in clusters
Explanation: This describes the appearance of Staphylococcus aureus. While Staph aureus can cause pneumonia, especially post-influenza or in IV drug users, it does not typically produce currant jelly sputum. Instead, it is more likely to cause abscesses or empyema.
CHOICE C: Gram-negative diplococci
Explanation: This describes bacteria like Neisseria gonorrhoeae or Neisseria meningitidis. These organisms are not typically associated with pneumonia in this clinical context and do not produce currant jelly sputum.
CHOICE D: Gram-negative rods with a thick capsule
Explanation: This describes Klebsiella pneumoniae, a gram-negative rod known for its thick polysaccharide capsule that gives it a mucoid appearance on culture. The patient's history of chronic alcohol use, aspiration risk, and currant jelly sputum are classic for Klebsiella pneumoniae.
CHOICE E: Gram-negative curved rods
Explanation: This describes organisms like Vibrio cholerae or Campylobacter jejuni, which are curved rods. These organisms are not associated with respiratory infections or currant jelly sputum.
FINAL VERDICT…
The most likely causative organism is Klebsiella pneumoniae, which is commonly associated with pneumonia in patients who have chronic alcohol use disorder and are prone to aspiration. The characteristic presentation includes fever, productive cough with currant jelly sputum, and consolidation on chest X-ray. The organism appears as gram-negative rods with a thick capsule on Gram stain, contributing to its ability to evade the immune system and cause severe necrotizing pneumonia.
KEY CONCEPTS:
The description of currant jelly sputum (thick, blood-tinged sputum) is highly suggestive of Klebsiella pneumoniae infection, which is strongly associated with chronic alcohol use and aspiration.
Klebsiella pneumoniae is a gram-negative rod with a thick capsule, which contributes to its virulence by preventing phagocytosis and allowing the formation of mucoid colonies.
CORRECT ANSWER: D) Gram-negative rods with a thick capsule
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