A 19-year-old student has two weeks of dry cough, low grade fever and bilateral patchy infiltrates out of proportion to her mild constitutional symptoms. Haemoglobin and leukocyte count are normal. The serological finding most supportive of the diagnosis is:

• A Positive Paul-Bunnell heterophile antibody test
• B Rise in cold agglutinin titre directed against adult I antigen on red cells ✓
• C ASO titre above 200 Todd units
• D Agglutination of Proteus OX-K at 1:160 dilution

Explanation

Mycoplasma pneumoniae causes atypical pneumonia with a nonproductive cough and disproportionate radiological findings. It induces IgM autoantibodies that agglutinate human O group red cells at 4 degrees C (cold agglutinins), positive in about half of cases. The Paul-Bunnell test points to infectious mononucleosis, OX-K to scrub typhus, and a raised ASO titre reflects streptococcal infection, none of which fits this picture.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.