Hypersensitivity and Autoimmunity · USMLE Step 1

A man develops a rash 10 days after starting a new antibiotic. Immune complex deposition is suspected. Which test helps confirm Type III hypersensitivity?

  1. Elevated IgE levels
  2. Low complement levels
  3. Positive ANA titer
  4. Peripheral eosinophilia
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Correct answer: Low complement levels

Immune complexes trigger complement consumption, leading to low C3/C4 levels. This supports a Type III mechanism.

Difficulty: Medium Question 11 of 20

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