**Core Concept:** Needle stick injury in healthcare workers, especially in high-risk patients like HIV patients, requires immediate post-exposure prophylaxis (PEP) for HIV prevention.

**Why the Correct Answer is Right:** The correct answer, **Option D:** HIV rapid test, is the most appropriate investigation in this scenario. Rapid HIV tests are highly sensitive and specific, and can provide results within 20-45 minutes. Post-exposure prophylaxis (PEP) must be initiated within 72 hours of exposure for maximum effectiveness.

**Why Each Wrong Option is Incorrect:**

A. Rapid plasma reagin (RPR) test - This test is not specific for HIV and has a high false-positive rate. It is not recommended for HIV diagnosis and is outdated.
B. HIV ELISA test - This test is time-consuming and requires a second test after four weeks to confirm the result. It is not suitable for immediate assessment of the risk of HIV infection.
C. VDRL test - Similar to RPR, this test is not specific for HIV and has a high false-positive rate. It is not recommended for HIV diagnosis.

**Why the Correct Option is Right:**

A rapid HIV test provides a rapid diagnosis, enabling immediate initiation of PEP and timely management. In contrast, the other options are either outdated, not specific for HIV, or too time-consuming for immediate decision-making.

**Clinical Pearl:** Healthcare workers should be trained to use personal protective equipment and adhere to proper needle stick injury protocols to minimize such incidents. If a needle stick injury occurs, immediate post-exposure prophylaxis (PEP) is crucial for the affected healthcare worker and should be initiated as soon as possible. The affected individual should be tested for HIV as soon as possible, preferably within 72 hours, and a confirmatory test should be performed within four weeks. This ensures a timely diagnosis and enables the initiation of effective PEP.