Widal test is used for susceptibility of

The correct option is Typhoid.

Step-by-step Explanation:

1. Understanding the Widal Test:
The Widal test is a classic serological assay developed by Georges-Fernand Widal in 1896. It is specifically designed to detect the presence of antibodies (agglutinins) in a patient's serum against the specific antigens of the bacterium Salmonella enterica serovar Typhi (commonly known as Salmonella Typhi), which causes typhoid fever.

2. How the Test Works:
The test measures agglutinating antibody titers against the O (somatic) and H (flagellar) antigens of Salmonella Typhi. When the patient's serum is mixed with these prepared bacterial antigens, the presence of matching antibodies causes the bacteria to clump together (agglutinate). A significant rise in the titer of these antibodies indicates an active infection or susceptibility to typhoid fever.

3. Evaluating the Options:
- Typhoid: Caused by Salmonella Typhi, it is the direct target of the diagnostic Widal test.
- Cholera: Caused by Vibrio cholerae, diagnosed primarily through stool culture or rapid diagnostic tests, not the Widal test.
- Malaria: A parasitic disease caused by Plasmodium species, diagnosed via microscopic examination of blood smears or rapid diagnostic tests (RDTs) detecting parasitic antigens.

Therefore, the Widal test is clinically utilized to diagnose and determine susceptibility to Typhoid.