A 60-year-old man on broad-spectrum antibiotics for pneumonia develops ecchymoses. PT is markedly prolonged, aPTT is mildly prolonged, and platelet count is normal. Which group of factors is vitamin K-dependent and affected in this patient?

• A Factors V, VIII, XI, XII
• B Factors II, VII, IX, X and proteins C and S ✓
• C Factors I, V, VIII, XIII
• D Factors VII, VIII, IX, XI

Explanation

Vitamin K is required for gamma-carboxylation of factors II, VII, IX, X and anticoagulant proteins C and S. Broad-spectrum antibiotics suppress gut flora that synthesize vitamin K, and warfarin blocks vitamin K epoxide reductase. Factors V, VIII, XI, XII, and XIII are not vitamin K-dependent.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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