This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Medicine
Among the following, earliest test to be positive in leptospirosis is: 
Select and explain. 
A. Microagglutination test (MAT), B. Leptospira culture of blood, C. IgM ELISA of serum, D. Macroagglutination test
C: IgM ELISA of serum
Answer: C: IgM ELISA of serum 
Explanation: Test for leptospirosis:Time to become positive:MAT(microscopic agglutination testing)Second week of illnessIgM ELISAWithin 3-5 daysBlood culture7-14 days after exposure(ref:Harrison's 17/e p1050)