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: Surgery
The development of a color reaction is indicative of a positive result in which of the following assay? 
Select and explain. 
A. Immunofluorescence, B. Flow cytometry, C. Radioimmunoassay, D. Enzyme- linked immunosorbent assay
D: Enzyme- linked immunosorbent assay
Answer: D: Enzyme- linked immunosorbent assay 
Explanation: ELISA is used routinely for screening patient sera for many different antibodies . Types :- 1. Indirect ELISA - uses color reaction. 2. Sandwhich ELISA - home pregnancy test's 3. Competitive ELISA-> ability to give results for cruder or samples. -Uses Enzyme linked Antigen rather than Enzyme linkedAntibody. -For detection of HIV Antibody.