A 30-year-old man presents with 10 days of malaise, dark urine, and right upper quadrant discomfort. AST 420 U/L, ALT 560 U/L, bilirubin 3.1 mg/dL. Serology shows HBsAg negative, anti-HBs negative, anti-HBc IgM positive, anti-HCV negative. Which phase of hepatitis B infection does this represent?
- A Window period of acute hepatitis B ✓
- B Early incubation period before antigenaemia
- C Recovered past infection with natural immunity
- D Chronic infection with HBsAg below detection threshold
Correct answer: A. Window period of acute hepatitis B
Explanation
The window period occurs after HBsAg has been cleared from blood but before anti-HBs becomes detectable. The only marker present is anti-HBc IgM, which confirms recent acute infection. Anti-HBs negativity excludes recovered immunity, where both anti-HBc IgG and anti-HBs are positive. During incubation, HBsAg would still be detectable. Anti-HBc IgM positivity is the key that distinguishes this from occult chronic infection, which shows anti-HBc IgG.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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