**Core Concept**
Haemorrhagic cystitis is a condition characterized by bleeding from the bladder mucosa, often resulting in pain and haematuria. It can be caused by various factors, including chemical irritants, radiation, and viral infections. The condition is usually associated with damage to the bladder lining, leading to ulceration and bleeding.

**Why the Correct Answer is Right**
Cyclophosphamide, a commonly used chemotherapeutic agent, is a well-known cause of haemorrhagic cystitis. This is due to its metabolite acrolein, which is a potent irritant to the bladder mucosa. Acrolein causes damage to the urothelial lining, leading to ulceration, bleeding, and eventually, haemorrhagic cystitis. The exact mechanism involves the binding of acrolein to tissue proteins, resulting in the formation of cytotoxic compounds that damage the bladder lining.

**Why Each Wrong Option is Incorrect**
**Option A:** Cyclophosphamide is not a cause of haemorrhagic cystitis; in fact, it is a known cause, making this option incorrect.
**Option B:** Radiation therapy can cause haemorrhagic cystitis, but it is not the only cause, and this option is too broad.
**Option C:** Viral infections, such as BK virus, can cause haemorrhagic cystitis, but this is not the most common or well-known cause.

**Clinical Pearl / High-Yield Fact**
To remember the causes of haemorrhagic cystitis, use the mnemonic "CARE": Cyclophosphamide, Acrolein, Radiation, and Eosinophilia (due to viral infections).

**Correct Answer:** C. Cyclophosphamide.