USMLE PRACTICE QUESTION #108
TOPIC: Microbiology/Histology
In USMLE practice question #108, we’ll test our ability to identify the cause of a commonly tested HEENT condition via microscopy/histology.
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A 3-year-old boy is brought to the pediatric clinic with fever, irritability, and ear pain. His mother reports he has been pulling on his left ear for the past two days. On examination, the child has a temperature of 38.9°C (102°F) and appears uncomfortable. Otoscopic examination reveals a bulging, erythematous tympanic membrane with decreased mobility. A culture is obtained from the middle ear fluid. Which of the following best describes the histological appearance of the organism most commonly responsible for this condition?
A) Gram-positive diplococci with a lancet shape
B) Gram-negative coccobacilli
C) Gram-positive cocci in clusters
D) Gram-negative diplococci with a kidney bean shape
E) Gram-positive bacilli in chains
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 best describes the histological appearance of the organism most commonly responsible for this condition?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 3rd order. 1st: Diagnose the condition; 2nd: Identify the most likely organism; 3rd: Identify the histological appearance of the likely organism.
Step 3: Read the vignette carefully and ask yourself: “What is the histological appearance of the most likely causative organism?”
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 3-year-old boy presenting with fever, irritability, and ear pain. He has been pulling on his left ear for the past two days, appears uncomfortable, and has a temperature of 38.9°C (102°F). Otoscopic examination reveals a bulging, erythematous tympanic membrane with decreased mobility—all of this points to a diagnosis of acute otitis media.
ANSWER CHOICES:
CHOICE A: Gram-positive diplococci with a lancet shape
Explanation: This shape describes Streptococcus pneumoniae, the most common bacterial cause of acute otitis media. It is responsible for about 40% to 80% of cases.
CHOICE B: Gram-negative coccobacilli
Explanation: This describes Haemophilus influenzae. While it can cause acute otitis media, it is not the most common causative organism.
CHOICE C: Gram-positive cocci in clusters
Explanation: Gram-positive cocci in clusters describe Staphylococcus aureus, which can cause various infections but is not a common cause of acute otitis media.
CHOICE D: Gram-negative diplococci with a kidney bean shape
Explanation: Gram-negative diplococci with a kidney bean shape describe Neisseria species, such as Neisseria gonorrhoeae or Neisseria meningitidis, which are not typically associated with acute otitis media.
CHOICE E: Gram-positive bacilli in chains
Explanation: Gram-positive bacilli in chains describe organisms like Bacillus or Clostridium species, which are not associated with acute otitis media.
FINAL VERDICT…
The most likely causative organism for this patient’s acute otitis media is Streptococcus pneumoniae, which appears as gram-positive diplococci with a characteristic lancet shape under microscopy. This bacterium is the most common pathogen responsible for acute otitis media in children.
CORRECT ANSWER: A) Gram-positive diplococci with a lancet shape
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