The correct answer is option 'D' - Widal Test is carried out to test for Typhoid fever.

Typhoid fever is a bacterial infection caused by the bacterium Salmonella typhi. It is primarily spread through contaminated food and water, and is more common in areas with poor sanitation and hygiene practices. The symptoms of typhoid fever include high fever, stomach pain, headache, weakness, and loss of appetite.

The Widal Test is a diagnostic test used to confirm the presence of antibodies against Salmonella typhi in a person's blood. It helps in the diagnosis of typhoid fever by detecting specific antibodies produced by the immune system in response to the infection. The test is based on the principle of agglutination, which is the clumping together of particles.

Here is a detailed explanation of the Widal Test:

1. Principle:
- The Widal Test is based on the agglutination reaction between the specific antibodies in the patient's blood and the antigens derived from Salmonella typhi.
- The test measures the presence and level of agglutinating antibodies, namely the O antigen (somatic antigen) and the H antigen (flagellar antigen), in the patient's serum.

2. Procedure:
- The patient's blood sample is collected and allowed to clot. The serum is then separated from the blood cells.
- A series of test tubes or wells are prepared, each containing specific dilutions of the antigens (O and H) derived from Salmonella typhi.
- The patient's serum is added to each test tube, and the tubes are observed for agglutination after incubation.
- Agglutination refers to the clumping together of the antigen-antibody complexes, indicating the presence of specific antibodies against Salmonella typhi.

3. Interpretation of Results:
- The Widal Test results are interpreted based on the presence and level of agglutination observed in each test tube.
- A significant rise in antibody titer (4-fold increase in dilution) between the acute and convalescent phase samples is considered indicative of a recent or current infection.
- The O antibody titer (anti-O) and H antibody titer (anti-H) are reported separately, and a high titer suggests a positive result for typhoid fever.

In conclusion, the Widal Test is a serological test used to detect antibodies against Salmonella typhi in the blood of individuals suspected of having typhoid fever. It plays a crucial role in the diagnosis and management of this infectious disease.

Typhoid fever