A 20-year-old student has fever, dry cough and bilateral interstitial infiltrates. Sputum shows no dominant organism and routine culture is negative. The cold agglutination titre of his serum is positive at 1:128. This result most strongly supports infection by:
- A Chlamydia psittaci
- B Mycoplasma pneumoniae ✓
- C Legionella pneumophila
- D Coxiella burnetii
Correct answer: B. Mycoplasma pneumoniae
Explanation
Cold agglutinins are IgM antibodies directed against the I antigen on erythrocytes, appearing in about half of patients with Mycoplasma pneumoniae pneumonia. A titre of 1:32 or higher, or a four-fold rise in paired samples, is considered supportive of the diagnosis. Legionella and Coxiella do not typically induce cold agglutinins, and Chlamydia psittaci produces a different serological profile with complement fixing antibodies.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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