**Core Concept**
The confirmatory test for HIV infection is a critical step in diagnosing the disease, as it differentiates between true-positive and false-positive results obtained from initial screening tests. The Western Blot test is a type of immunoblotting assay that detects specific antibodies against HIV proteins, providing a more definitive diagnosis.

**Why the Correct Answer is Right**
The Western Blot test is a confirmatory test for HIV infection because it involves the separation of HIV proteins by size using electrophoresis, followed by the transfer of these proteins to a membrane. Antibodies against HIV in the patient's serum are then detected using enzyme-linked immunosorbent assay (ELISA) or chemiluminescence. The presence of specific antibodies against HIV proteins, such as gag, pol, and env, confirms the diagnosis of HIV infection. This test is highly specific and sensitive, making it the gold standard for confirming HIV infection.

**Why Each Wrong Option is Incorrect**
**Option A:** ELISA (Enzyme-Linked Immunosorbent Assay) is a screening test for HIV infection, not a confirmatory test. While it is highly sensitive, it can produce false-positive results, which need to be confirmed by a more specific test like Western Blot.

**Option B:** Immunodot is a type of rapid diagnostic test that detects antibodies against HIV, but it is not a confirmatory test. It is often used in resource-limited settings due to its simplicity and rapid results, but it lacks the specificity and sensitivity of Western Blot.

**Option C:** RIPA (Radioimmunoprecipitation Assay) is a molecular biology technique used to detect specific proteins or antibodies, but it is not typically used as a confirmatory test for HIV infection. It is more commonly used in research settings to study protein-protein interactions.

**Clinical Pearl / High-Yield Fact**
The Western Blot test is the gold standard for confirming HIV infection, but it is not 100% sensitive or specific. A negative result does not rule out HIV infection, especially in individuals with a high risk of exposure. Therefore, a confirmatory test should always be performed in conjunction with clinical evaluation and patient history.

**✓ Correct Answer: D. Western Blot**