**Core Concept**
The Widal test is a serological test used to diagnose typhoid fever caused by Salmonella Typhi. It measures the level of antibodies against the O and H antigens of the bacteria.

**Why the Correct Answer is Right**
The Widal test detects the presence of agglutinins, specifically IgM and IgG antibodies, against the O and H antigens of Salmonella Typhi. The test involves mixing the patient's serum with suspensions of O and H antigens. If the patient has been infected with Salmonella Typhi, their serum will contain antibodies that react with the antigens, causing visible agglutination. The test is typically performed on the 7th to 14th day of illness and is considered positive if the tire (dilution) of the antibodies is 1:80 or higher against either the O or H antigen.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Widal test is not a rapid diagnostic test and requires a laboratory setting for performance.
**Option B:** This option is incorrect because the Widal test does not detect the presence of Salmonella Typhi bacteria itself, but rather the antibodies produced in response to infection.
**Option C:** This option is incorrect because the Widal test does not differentiate between Salmonella Typhi and other Salmonella species.

**Clinical Pearl / High-Yield Fact**
The Widal test is a classic example of a serological test used in the diagnosis of infectious diseases. It is essential to remember that a positive Widal test requires clinical correlation, as it can be influenced by various factors such as recent vaccination or cross-reactivity with other Salmonella species.

**Correct Answer:**