Welcome to USMLE practice question #138! Today’s topic is microbiology.
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A 6-year-old girl presents with a 3-day history of fever, sore throat, and a diffuse rash. Physical examination reveals erythema of the oropharynx with tonsillar exudates, cervical lymphadenopathy, and a fine, sandpaper-like rash on her chest and extremities. A throat swab grows a gram-positive coccus in chains that is catalase-negative and demonstrates β-hemolysis on blood agar. Which of the following virulence factors is most likely responsible for the rash observed in this patient?
A) Streptolysin O
B) Streptokinase
C) M protein
D) Erythrogenic exotoxin
E) Hyaluronidase
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 virulence factors is most likely responsible for the rash observed in this patient?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order. 1st: Identify the organism based on the given information, 2nd: Identify how that organism causes a rash.
Step 3: Read the vignette carefully and ask yourself: “Based on the given information and my recognition of the organism, the most likely mechanism underlying this patient’s rash is ______________.”
Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.
GENERAL ANALYSIS
This 6-year-old girl presents with fever, sore throat, cervical lymphadenopathy, and a sandpaper-like rash, which are classic features of scarlet fever. The causative organism is identified as Streptococcus pyogenes (Group A Streptococcus, GAS), a Gram-positive, catalase-negative, β-hemolytic coccus.
ANSWER CHOICES:
CHOICE A: Streptolysin O
Explanation: Streptolysin O is a hemolysin produced by S. pyogenes that causes lysis of red and white blood cells. It contributes to the pathogenicity of GAS but does not cause the erythematous rash seen in scarlet fever.
CHOICE B: Streptokinase
Explanation: Streptokinase is an enzyme that activates plasminogen to plasmin, promoting fibrinolysis and aiding in tissue invasion by GAS. While streptokinase enhances bacterial spread, it does not play a role in the development of the rash in scarlet fever.
CHOICE C: M protein
Explanation: M protein is a major virulence factor of S. pyogenes that inhibits phagocytosis and aids in immune evasion. It is associated with post-streptococcal complications like rheumatic fever.
CHOICE D: Erythrogenic exotoxin
Explanation: Erythrogenic exotoxins (also called streptococcal pyrogenic exotoxins or SPEs) are superantigens produced by certain strains of S. pyogenes. These toxins cause widespread activation of T cells and cytokine release, leading to capillary damage.
CHOICE E: Hyaluronidase
Explanation: Hyaluronidase is an enzyme produced by S. pyogenes that degrades hyaluronic acid in connective tissue, facilitating bacterial spread through tissues.
FINAL VERDICT…
CORRECT ANSWER: D) Erythrogenic exotoxin
The characteristic sandpaper-like rash in scarlet fever is caused by erythrogenic exotoxins produced by certain strains of Streptococcus pyogenes. These toxins act as superantigens, inducing widespread inflammation and capillary damage—the direct cause of the rash observed in this patient.
KEY CONCEPTS:
Scarlet Fever Overview:
Caused by S. pyogenes strains that produce erythrogenic exotoxins.
Presents with sore throat, fever, cervical lymphadenopathy, and a sandpaper-like rash.
Rash typically begins on the chest and spreads to the extremities.
Role of Erythrogenic Exotoxins (SPEs):
Superantigens that activate large numbers of T cells.
Cause cytokine release, capillary damage, and erythema.
Responsible for the rash, "strawberry tongue," and desquamation seen in scarlet fever.
Clinical Features of Scarlet Fever Rash:
Diffuse erythematous rash with a sandpaper texture.
Often starts on the chest and spreads outward.
Spares the palms and soles.
May be accompanied by "strawberry tongue."
Diagnosis and Treatment:
Diagnosis: Rapid strep test or throat culture.
Treatment: Penicillin or amoxicillin to eradicate S. pyogenes.