MCQ of the Day: 25 October 2026

Answer: B. Microscopic agglutination test. Fever, calf myalgia, conjunctival suffusion, jaundice and acute kidney injury after floodwater exposure in the monsoon is classic for severe leptospirosis (Weil's disease), caused by Leptospira interrogans shed in rodent urine. The microscopic agglutination test (MAT) is the reference serological method: patient serum is mixed with a panel of live leptospiral serovars and agglutination is read under dark-field microscopy. A fourfold rise in titre between paired sera confirms infection. MAT needs live cultures and is mostly done in reference laboratories, and it is often negative in the first week, when blood PCR or culture (for example in EMJH medium) is more useful; IgM ELISA is widely used for screening.
A. Weil-Felix uses cross-reacting Proteus OX antigens to detect rickettsial antibodies, as in scrub typhus (OX-K); despite the name, it is not used for Weil's disease.
C. Widal detects agglutinins to Salmonella Typhi O and H antigens in enteric fever.
D. Paul-Bunnell detects heterophile antibodies in infectious mononucleosis due to Epstein-Barr virus.