A labourer involved with repair work of sewers was admitted with fever, jaundice and renal failure. The most appropriate test to diagnose the infection in this patient is:

This patient is suffering from Weil's disease - the most severe form of leptospirosis characterised by fever, jaundice and renal failure. Diagnosis of leptospirosis can be made by the gold standard microscopic agglutination test (MAT). The Weil - Felix test is an agglutination test for the diagnosis of rickettsial infections. Paul Bunnel test is for the heterophile antibodies used in the diagnosis of infectious mononucleosis. Ref: Harrisons principles of internal medicine, 18th edition, chapter: 171