## **Core Concept**
The patient's symptoms of sore throat, diarrhea, and a history of sexual contact 2 weeks prior suggest a potential acute retroviral syndrome, which is a common presentation of acute HIV infection. The window period for HIV testing is crucial as it refers to the time between potential exposure and when a test can reliably detect the presence of the virus.

## **Why the Correct Answer is Right**
The best investigation to rule out HIV at this stage would be the **4th generation HIV-1/HIV-2 antigen/antibody (Ag/Ab) test**. This test detects both the p24 antigen of the virus and antibodies against HIV, allowing for earlier detection than antibody-only tests. Given that the patient is 2 weeks post-exposure, this test is most likely to provide an accurate result as it can detect the p24 antigen before the body has produced antibodies.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **Western blot test** is a confirmatory test used after a positive screening test. It is not used for initial screening due to its lower sensitivity and higher cost.
- **Option B:** The **ELISA (Enzyme-Linked Immunosorbent Assay) test** for antibodies only may not detect early infections since it relies on the body's immune response, which can take time to develop.
- **Option D:** **Rapid tests** are convenient but may have lower sensitivity compared to laboratory-based tests like the 4th generation Ag/Ab test, especially in early infection.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is the **window period** for HIV testing. For most tests, it's around 3-4 weeks, but with the 4th generation Ag/Ab test, the window period is approximately 2 weeks. Patients with a high risk of exposure should be tested at baseline and at 3 months post-exposure if initial test is negative.

## **Correct Answer:** . 4th generation HIV-1/HIV-2 antigen/antibody (Ag/Ab) test