**Core Concept**
Sterile haemorrhagic cystitis is a condition characterized by bleeding from the bladder mucosa in the absence of bacterial infection. It is often associated with certain medications or treatments that cause direct damage to the bladder lining.

**Why the Correct Answer is Right**
The correct answer is C. Cyclophosphamide. Cyclophosphamide is a chemotherapeutic agent that can cause sterile haemorrhagic cystitis due to its metabolite acrolein, which directly damages the bladder mucosa. This damage leads to bleeding and inflammation of the bladder lining, resulting in haemorrhagic cystitis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify a known causative agent of sterile haemorrhagic cystitis. While certain medications may cause bladder damage, they are not as directly associated with this condition as cyclophosphamide.

**Option B:** This option is incorrect as it refers to a condition that is not directly related to sterile haemorrhagic cystitis. Interstitial cystitis is a separate condition characterized by chronic bladder pain and inflammation, often without bleeding.

**Option D:** This option is incorrect as it is not a known causative agent of sterile haemorrhagic cystitis. While certain medications may cause bladder damage, they are not as directly associated with this condition as cyclophosphamide.

**Clinical Pearl / High-Yield Fact**
It is essential to note that cyclophosphamide-induced haemorrhagic cystitis is more likely to occur with high doses or prolonged treatment. Patients receiving cyclophosphamide should be monitored for signs of bladder toxicity, and preventive measures such as mesna administration may be considered.

**Correct Answer:** C. Cyclophosphamide.