This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
K 39 antigen is used in diagnosis of: 
Select and explain. 
A. Malaria, B. Typhus fever, C. Kala azar, D. Enteric fever
C: Kala azar
Answer: C: Kala azar 
Explanation: Rapid Diagnostic Test: Now a rapid dipstick test based on the recombinant K39 protein is available for rapid diagnosis of kala-azar. K39 is an epitope apparently conserved on amastigotes of Leishmania species that cause visceral infection; by use of laboratory ELISA testing, circulating anti-K39, IgG is detectable in 95%- 100% of patients who have kala-azar, irrespective of geographic region. Ref: