**Core Concept**
The Widal test is a serological test used for the diagnosis of typhoid fever, which is caused by the bacterium *Salmonella Typhi*. This test detects the presence of antibodies against the O and H antigens of *S. Typhi* in the patient's serum.

**Why the Correct Answer is Right**
The Widal test is a type of agglutination test, which detects the presence of antibodies that cause the clumping of red blood cells or bacterial antigens. In this case, the test detects the presence of antibodies against the O and H antigens of *S. Typhi*. The O antigen is a lipopolysaccharide component of the bacterial cell wall, while the H antigen is a flagellar protein. The presence of antibodies against these antigens indicates that the patient has been infected with *S. Typhi*. The test involves mixing the patient's serum with suspensions of *S. Typhi* O and H antigens, and observing for agglutination.

**Why Each Wrong Option is Incorrect**
**Option A:** The Widal test is not a type of PCR (Polymerase Chain Reaction), which is a molecular diagnostic technique used to amplify specific DNA sequences.
**Option B:** The Widal test is not a type of ELISA (Enzyme-Linked Immunosorbent Assay), which is a serological test used to detect the presence of antibodies or antigens in a sample.
**Option C:** The Widal test is not a type of culture test, which involves growing the bacteria in a laboratory setting to diagnose infection.

**Clinical Pearl / High-Yield Fact**
The Widal test is most useful in the second week of illness, when the antibody response is maximal. However, a negative test in the first week does not rule out typhoid fever, as the antibody response may take time to develop.

**Correct Answer: D. Agglutination test**