**Core Concept**
Cyclophosphamide, a commonly used chemotherapeutic agent, can cause hemorrhagic cystitis as a side effect due to its metabolite acrolein. This condition occurs when the chemotherapy agent or its metabolites directly damage the bladder mucosa, leading to inflammation and ulceration.

**Why the Correct Answer is Right**
The correct answer is **Cyclophosphamide**. This is because cyclophosphamide is a nitrogen mustard alkylating agent that is used in the treatment of various cancers, including lymph-node-positive breast cancer. When administered, it is metabolized to 4-hydroxycyclophosphamide, which is then converted to acrolein and phosphoramide mustard. Acrolein is the compound responsible for causing hemorrhagic cystitis by directly damaging the bladder mucosa and leading to inflammation and ulceration.

**Why Each Wrong Option is Incorrect**
* **Option A:** Folinic acid (leucovorin) is used to prevent or treat methotrexate toxicity, not cyclophosphamide toxicity. Methotrexate is a different class of chemotherapeutic agents called antifolates.
* **Option B:** Mesna is used to prevent hemorrhagic cystitis caused by ifosfamide, not cyclophosphamide. Ifosfamide is another nitrogen mustard alkylating agent that is structurally similar to cyclophosphamide but has a different side effect profile.
* **Option D:** Probenecid is used to prevent gout attacks by reducing uric acid reabsorption in the kidneys. It is not related to the prevention of hemorrhagic cystitis caused by cyclophosphamide.

**Clinical Pearl / High-Yield Fact**
To prevent hemorrhagic cystitis caused by cyclophosphamide, patients can be treated with Mesna, which directly neutralizes acrolein and prevents bladder toxicity. This is an important consideration in patients receiving high-dose cyclophosphamide therapy.

**Correct Answer:** C. Cyclophosphamide