Which of the following chemical tests is used in the diagnosis of typhoid?

Options

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

Answer

To answer the question regarding which chemical test is used in the diagnosis of typhoid, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for the specific chemical test used to diagnose typhoid fever.

2. **Identify the Options**: The options provided are:
- Option 1: ELISA
- Option 2: ESR
- Option 3: PCR
- Option 4: Vidal test

3. **Evaluate Each Option**:
- **Option 1: ELISA**: This test stands for Enzyme-Linked Immunosorbent Assay. It is primarily used for detecting antibodies related to HIV/AIDS. Therefore, it is not used for diagnosing typhoid.

- **Option 2: ESR**: ESR stands for Erythrocyte Sedimentation Rate. This test measures the rate at which red blood cells settle in a tube over a specified period. It is not specific for typhoid diagnosis but can indicate inflammation or infection in the body.

- **Option 3: PCR**: Polymerase Chain Reaction (PCR) is a technique used to amplify DNA and can be used for various infections, including some viral infections. However, it is not the standard test for typhoid.

- **Option 4: Vidal test**: The Vidal test is specifically designed to detect the presence of antibodies against Salmonella typhi, the bacteria responsible for typhoid fever. This makes it the correct test for diagnosing typhoid.

4. **Conclusion**: Based on the evaluation, the correct answer is **Option 4: Vidal test**.

### Final Answer:
The chemical test used in the diagnosis of typhoid is the **Vidal test**.