**Core Concept**
The patient's symptoms of fever, cervical lymphadenopathy, and night sweats four weeks after unprotected sexual contact are suggestive of acute HIV infection. Early diagnosis of HIV requires the detection of viral antigens or genetic material.

**Why the Correct Answer is Right**
The p24 antigen test detects the core protein p24 of the HIV virus, which is present in the blood during the acute phase of infection, typically 14-28 days after exposure. This test is particularly useful during the "window period" when the virus is not yet detectable by other methods like ELISA or HIV PCR. The p24 antigen test has a high sensitivity and specificity, making it an ideal choice for early diagnosis of HIV.

**Why Each Wrong Option is Incorrect**
**Option B:** ELISA (Enzyme-Linked Immunosorbent Assay) is a serological test that detects antibodies against HIV. However, it may not be sensitive during the early stages of infection, as it takes time for the body to produce antibodies. ELISA is typically used for confirmatory testing after a positive result from a rapid test.

**Option C:** CD4 count is a measure of the patient's immune status, specifically the number of CD4 T cells in the blood. While a low CD4 count is indicative of advanced HIV disease, it is not a diagnostic test for HIV infection.

**Option D:** HIV PCR (Polymerase Chain Reaction) is a molecular test that detects the genetic material of HIV. However, PCR is typically used for viral load monitoring in established HIV infection, rather than early diagnosis.

**Clinical Pearl / High-Yield Fact**
In patients with suspected acute HIV infection, a negative p24 antigen test does not rule out HIV, as the test may not be sensitive during the early stages of infection. A repeat test in 14-28 days may be necessary to confirm the diagnosis.

**✓ Correct Answer: A. p24 antigen test.**