**Question:** Which one of the following can cause Agranulocytosis most commonly?

A. Azathioprine
B. Allopurinol
C. Trimethoprim
D. Warfarin

**Core Concept:** Agranulocytosis is a severe and rare hematological complication characterized by a significant decrease in granulocytes (neutrophils, basophils, and eosinophils) in the bloodstream. Granulocytes play a crucial role in fighting infections, and their deficiency increases the risk of severe bacterial, fungal, and viral infections.

**Why the Correct Answer is Right:** Azathioprine is an immunosuppressive drug commonly used to prevent organ transplant rejection and treat certain autoimmune disorders. It works by inhibiting the proliferation of lymphocytes (T cells and B cells) and myeloid cells, leading to a decrease in granulocyte production. This results in agranulocytosis as a potential side effect, making it the correct answer.

**Why Each Wrong Option is Incorrect:**

B. Allopurinol is an antimetabolite used to treat hyperuricemia and gout. Although it can cause leukopenia (a decrease in total white blood cell count), agranulocytosis is a rare side effect.

C. Trimethoprim is an antibiotic used to treat urinary tract infections and prophylaxis of Pneumocystis pneumonia in patients with HIV. Although it can cause leukopenia, agranulocytosis is a rare side effect.

D. Warfarin is an anticoagulant used to prevent blood clots and manage deep vein thrombosis. While it can cause leukopenia and thrombocytopenia, agranulocytosis is not a common side effect.

**Clinical Pearl:** Agranulocytosis is a severe side effect and a potential life-threatening complication. Patients on immunosuppressive medications like Azathioprine should be monitored closely for their complete blood count, including granulocytes, to promptly detect any decrease. Early intervention may prevent severe infections and their complications.