A 19-year-old college student has 10 days of fever, malaise and a persistent dry cough. Chest examination reveals scant crackles although the radiograph shows extensive patchy lower-zone infiltrates, a disparity noted repeatedly. Haemoglobin is 9 g/dL with reticulocytosis, and the cold agglutinin titre is 1:256. Which organism is the most likely cause?

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

Explanation

Mycoplasma pneumoniae classically produces walking pneumonia with physical signs far milder than the radiographic infiltrates, plus extrapulmonary features including cold agglutinin mediated autoimmune haemolytic anaemia, erythema multiforme and encephalitis. Legionella causes hyponatraemia and gastrointestinal symptoms, psittaci requires bird exposure, and pneumococcus produces lobar consolidation with prominent focal signs rather than cold agglutinins.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.