**Question:** Azidothymidine (Zidovudine), commonest side effect is

A. Osteonecrosis
B. Hepatitis
C. Myelosuppression
D. Neuropathy

**Correct Answer:** C. Myelosuppression

**Core Concept:**
Azidothymidine (Zidovudine) is an antiviral drug primarily used in the treatment of HIV infection, specifically as part of combination antiretroviral therapy (ART). It belongs to the class of drugs known as nucleoside reverse transcriptase inhibitors (NRTIs). The primary mechanism of action involves inhibiting reverse transcriptase, which is an essential enzyme in the replication of the Human Immunodeficiency Virus (HIV). However, this drug has some side effects associated with its use.

**Why the Correct Answer is Right:**
Among the given options, the most common side effect of Zidovudine is **Myelosuppression**. This refers to the suppression of various components of the blood, specifically the production of white blood cells, red blood cells, and platelets. As a result, patients may experience symptoms such as fatigue, anemia, and increased susceptibility to infections.

**Why Each Wrong Option is Incorrect:**

A. Osteonecrosis: This refers to bone death, typically due to decreased blood supply. While Zidovudine can cause bone marrow toxicity, osteonecrosis is not a direct side effect of the drug.

B. Hepatitis: Although Zidovudine has been associated with liver toxicity, it is not the most common side effect and is not mentioned as the most common side effect in the question.

D. Neuropathy: While Zidovudine can cause peripheral neuropathy, it is not the most common side effect, and the question specifies that we are looking for the most common side effect.

**Clinical Pearl:**
Myelosuppression is an important side effect to recognize due to its potential implications on a patient's immune system and overall health. It is essential for clinicians to monitor patients on Zidovudine therapy and adjust their dosage or switch to alternative therapies if necessary.