The chemical test that is used for diagnosis of typhoid is

Options

A. ELISA test
B. ESR test
C. PCR test
D. Widal test

Explanation

**Step-by-Step Text Solution:**

1. **Identify the Disease**: The question asks about the chemical test used for diagnosing typhoid, which is caused by the bacterium *Salmonella typhi*.

2. **Recognize the Diagnostic Test**: The primary test used for diagnosing typhoid fever is known as the Widal test.

3. **Understand the Mechanism of the Widal Test**: The Widal test is an indirect agglutination test. This means it detects the presence of antibodies (specifically IgM and IgG) in the serum of an infected person.

4. **Explain the Reaction**: In the Widal test, the serum from the patient is mixed with specific antigens from *Salmonella typhi*. If the antibodies are present, they will bind to the antigens, leading to agglutination (clumping) of the bacteria.

5. **Interpret the Results**: A positive Widal test indicates the presence of antibodies against *Salmonella typhi*, suggesting that the person may be infected with typhoid fever.

**Final Answer**: The chemical test used for the diagnosis of typhoid is the Widal test.

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