A 19-year-old college student develops pharyngitis followed by a persistent dry cough and wheeze. Chest radiograph shows patchy perihilar infiltrates that look far worse than his scanty signs suggest. Haemoglobin has fallen from 13.2 to 10.8 g/dL with reticulocytosis, and the cold agglutinin titre is markedly elevated. Which organism is the most likely cause?

• A Legionella pneumophila
• B Mycoplasma pneumoniae ✓
• C Chlamydophila psittaci
• D Coxiella burnetii

Explanation

Mycoplasma pneumoniae classically produces tracheobronchitis with disproportionate radiographic findings, and IgM antibodies against the I antigen generate cold agglutinins, causing autoimmune haemolytic anaemia in a minority of cases. Legionella and Coxiella produce systemic illness with relative bradycardia, hyponatraemia and gastrointestinal features but are not linked to cold agglutinin haemolysis. Psittaci requires bird exposure. The combination of pharyngitis, atypical radiology and haemolysis is essentially diagnostic of Mycoplasma.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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