**Question:** Following drugs are known immunosuppressive agent except -

A. Cyclophosphamide
B. Azathioprine
C. Prednisolone
D. Allopurinol

**Correct Answer:** D. Allopurinol

**Core Concept:** Immunosuppressive agents are medications that reduce the activity of the immune system to prevent organ transplant rejection or treat autoimmune diseases. They work by suppressing the activity of immune cells, cytokines, and other components involved in immune response.

**Why the Correct Answer is Right:** Allopurinol is primarily an anti-hyperuricemia drug, used to lower uric acid levels in patients with gout or hyperuricemia. It works by inhibiting xanthine oxidase, which reduces the production of uric acid. Allopurinol does not have immunosuppressive properties, making it an exception among the given options.

**Why Each Wrong Option is Incorrect:**

A. Cyclophosphamide: This is a cytotoxic drug used to treat various cancers and autoimmune disorders. It works by inhibiting DNA synthesis and inducing apoptosis in rapidly dividing cells, including cancer cells and immune cells, thereby suppressing the immune system.

B. Azathioprine: This drug is an immunosuppressive agent used for the same indications as cyclophosphamide. It works by inhibiting DNA synthesis and is a prodrug converted to 6-mercaptopurine, which further suppresses the immune system.

C. Prednisolone: This drug is a glucocorticoid with potent anti-inflammatory and immunosuppressive properties. It works by binding to glucocorticoid receptors and modulating gene expression, ultimately suppressing immune cell function and inflammation.

**Clinical Pearl:** When prescribing immunosuppressive agents, it is crucial to consider potential adverse effects, including increased risk of infections, malignancies, and secondary organ damage. Always weigh the benefits and risks before employing immunosuppressive therapies.