USMLE PRACTICE QUESTION #104
TOPIC: Dermatology
Question #104 will test your understanding of a highly-tested USMLE Step 1 dermatology topic.
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A 68-year-old male with a history of diabetes mellitus and hypertension presents to the clinic with complaints of severe, burning pain on the right side of his chest. The pain began three days ago and was followed by the appearance of a vesicular rash localized to the T4 dermatome. He reports no recent illness and has never had a similar rash before. Which of the following diagnostic findings is most likely in this patient?
A) Tzanck smear showing multinucleated giant cells
B) Detection of heterophile antibodies
C) Positive rapid plasma reagin (RPR) test
D) Gram stain showing gram-positive cocci
E) IgM antibody response against cytomegalovirus
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 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 diagnostic findings is most likely in this patient?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order. 1st: Diagnose the condition; 2nd: Recognize the diagnostic finding associated with this condition.
Step 3: Read the vignette carefully and ask yourself: “Based on my diagnosis, which of the findings is most likely?”
Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.
GENERAL ANALYSIS
A 68-year-old male presents with severe, burning pain on the right side of his chest, followed by a vesicular rash localized to the T4 dermatome. The patient has a history of diabetes mellitus and hypertension. The clinical presentation strongly suggests herpes zoster (shingles), caused by varicella-zoster virus (VZV) reactivation. The question asks for the most likely diagnostic finding.
ANSWER CHOICES:
CHOICE A: Tzanck smear showing multinucleated giant cells
Explanation: A Tzanck smear is a cytological test that can detect herpesvirus infections, including varicella-zoster virus (VZV) and herpes simplex virus (HSV). The smear shows multinucleated giant cells, which are characteristic of herpesvirus infections.
CHOICE B: Detection of heterophile antibodies
Explanation: Heterophile antibodies are detected in patients with infectious mononucleosis caused by the Epstein-Barr virus (EBV). This patient’s presentation is consistent with herpes zoster, not infectious mononucleosis.
CHOICE C: Positive rapid plasma reagin test
Explanation: The RPR test is used to screen for syphilis, which can cause various systemic symptoms but does not typically present with a dermatomal vesicular rash.
CHOICE D: Gram stain showing gram-positive cocci
Explanation: Gram-positive cocci are associated with bacterial infections like Staphylococcus aureus or Streptococcus species. This patient’s presentation suggests a viral infection (herpes zoster).
CHOICE E: IgM antibody response against cytomegalovirus
Explanation: Cytomegalovirus causes systemic infections, particularly in immunocompromised individuals, but does not typically cause a vesicular rash in a dermatomal distribution.
FINAL VERDICT…
The patient's clinical presentation—burning pain followed by a vesicular rash localized to the T4 dermatome—is classic for herpes zoster (shingles). Herpes zoster results from reactivation of the varicella-zoster virus (VZV), which remains latent in dorsal root ganglia after primary infection (chickenpox). In older adults or immunocompromised individuals, VZV can reactivate and cause shingles.
A Tzanck smear from a vesicle can show characteristic findings such as multinucleated giant cells, which are indicative of herpesvirus infections (including VZV). Although PCR testing has largely replaced the Tzanck smear for diagnosis due to its higher sensitivity and specificity, the Tzanck smear remains an important historical diagnostic tool that can confirm VZV infection.
Thus, the most likely diagnostic finding in this patient would be multinucleated giant cells on a Tzanck smear.
CORRECT ANSWER: A) Tzanck smear showing multinucleated giant cells
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That’s it for question 104!
See ya tomorrow 👋