**Core Concept**
Hemorrhagic cystitis is a condition characterized by bleeding from the bladder mucosa, often resulting in hematuria. It is typically caused by chemical or radiation damage to the bladder lining. The condition can be acute or chronic, with various etiologies including medications, infections, and radiation therapy.

**Why the Correct Answer is Right**
The correct answer is **Cyclophosphamide**. Cyclophosphamide is a chemotherapeutic agent that belongs to the class of nitrogen mustard alkylating agents. When metabolized, it produces acrolein, a toxic compound that directly damages the bladder mucosa, leading to hemorrhagic cystitis. This condition is a well-documented side effect of cyclophosphamide treatment, and patients often require supportive measures to manage the symptoms.

**Why Each Wrong Option is Incorrect**
* **Option A:** Cisplatin is another chemotherapeutic agent, but it is not primarily associated with hemorrhagic cystitis. While it can cause renal toxicity, the mechanism is different from cyclophosphamide.
* **Option B:** Bacillus Calmette-Guerin (BCG) is a live attenuated vaccine used in the treatment of bladder cancer. Although it can cause a range of side effects, hemorrhagic cystitis is not a common association.
* **Option D:** E. coli is a common cause of urinary tract infections, but it is not typically associated with hemorrhagic cystitis. While certain infections can cause cystitis, the condition described is more closely related to chemical or radiation damage.

**Clinical Pearl / High-Yield Fact**
Cyclophosphamide-induced hemorrhagic cystitis can be mitigated with the use of mesna, a sulfhydryl compound that binds to acrolein and reduces its toxic effects on the bladder mucosa.

**Correct Answer:** C. Cyclophosphamide