Infection by a pathogen can be detected by the presence of antigens or by detecting the antibody synthesized against the pathogen. The test/s based on this principle is/are

Options

A. PCR
B. ELISA
C. Both PCR and ELISA
D. Neither PCR nor ELISA

Answer

To solve the question regarding the detection of infection by a pathogen through the presence of antigens or antibodies, we will analyze the tests mentioned: PCR and ELISA.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about tests that can detect infections by identifying either the antigens of the pathogen or the antibodies produced against it.

2. **Identifying the Tests**: The two tests mentioned are:
- **PCR (Polymerase Chain Reaction)**: This is a technique used to amplify DNA sequences. It is primarily used for detecting the genetic material of pathogens, not specifically for detecting antigens or antibodies.
- **ELISA (Enzyme-Linked Immunosorbent Assay)**: This test is designed to detect the presence of antibodies or antigens in a sample. It works on the principle of antigen-antibody interaction.

3. **Analyzing PCR**: Since PCR amplifies DNA and is not based on the detection of antigens or antibodies, it does not fit the criteria of the question. Therefore, PCR is not suitable for detecting infections based on antigen-antibody interactions.

4. **Analyzing ELISA**: ELISA is specifically designed to detect either antigens from the pathogen or antibodies produced by the immune system in response to the pathogen. It is a direct method of identifying infections based on the presence of these molecules.

5. **Conclusion**: Based on the analysis, the test that can detect infection by the presence of antigens or antibodies is ELISA. PCR does not meet the criteria set by the question.

### Final Answer:
The correct test based on the principle of detecting infection by the presence of antigens or antibodies is **ELISA**.