**Core Concept**
The accidental exposure to HIV-positive blood through a needle stick poses a significant risk of HIV transmission. The decision to administer post-exposure prophylaxis (PEP) depends on the viral load of the source patient and the promptness of initiation of treatment. In this scenario, the source patient has a high viral load, which increases the risk of HIV transmission.

**Why the Correct Answer is Right**
The correct answer involves administering a combination of antiretroviral medications to prevent HIV infection. Zidovudine (AZT) is a nucleoside reverse transcriptase inhibitor (NRTI) that has been shown to be effective in reducing the risk of HIV transmission when administered promptly after exposure. Lamivudine (3TC) is another NRTI that is often used in combination with zidovudine to enhance its efficacy. The combination of zidovudine and lamivudine has been demonstrated to be effective in preventing HIV infection when initiated within 72 hours of exposure. In this scenario, a 4-week course of treatment is recommended to ensure that the nurse is adequately protected against HIV infection.

**Why Each Wrong Option is Incorrect**
**Option A:** Monitoring the nurse's blood to determine whether HIV transmission has occurred or not is not an appropriate response, as it may be too late to prevent HIV infection once the virus has established itself in the body. Prompt initiation of PEP is crucial to prevent HIV infection.

**Option B:** Treating the nurse with full doses of zidovudine for 2 weeks is not sufficient to prevent HIV infection, as the duration of treatment may be too short to effectively reduce the risk of transmission.

**Option C:** Adding acyclovir to a 4-week zidovudine regimen is not necessary, as acyclovir is an antiviral medication used to treat herpes simplex virus (HSV) infections, not HIV. This combination is not recommended for PEP.

**Clinical Pearl / High-Yield Fact**
When a healthcare worker is exposed to HIV-positive blood, prompt initiation of post-exposure prophylaxis (PEP) within 72 hours is crucial to prevent HIV infection. A combination of antiretroviral medications, such as zidovudine and lamivudine, is recommended for a 4-week course to ensure adequate protection against HIV infection.

**✓ Correct Answer: D. Administer zidovudine with lamivudine for 4weeks.**