**Core Concept**
Oral administration is a convenient and non-invasive route for delivering medications, but not all chemotherapeutic agents can be given orally due to their pharmacokinetic properties. The oral bioavailability and stability of a drug are crucial factors in determining its suitability for oral administration.

**Why the Correct Answer is Right**
Mesna (sodium 2-sulfanylethanesulfonate) is a unique chemoprotective agent used primarily to prevent hemorrhagic cystitis caused by ifosfamide and cyclophosphamide. Its oral bioavailability is approximately 90%, making it an ideal candidate for oral administration. Mesna works by conjugating with acrolein, a toxic metabolite of ifosfamide and cyclophosphamide, thereby preventing its urotoxic effects.

**Why Each Wrong Option is Incorrect**
**Option A:** Cytosine arabinoside is a pyrimidine analog used in the treatment of leukemia. It is not suitable for oral administration due to its poor oral bioavailability and rapid first-pass metabolism in the liver.
**Option B:** Cisplatin is a platinum-based chemotherapeutic agent used to treat various types of cancer. It is highly toxic and has a low oral bioavailability, making it unsuitable for oral administration.
**Option C:** Doxorubicin is an anthracycline antibiotic used to treat cancer. It is not suitable for oral administration due to its poor oral bioavailability and potential for severe cardiotoxicity.

**Clinical Pearl / High-Yield Fact**
Mesna is a chemoprotective agent that can prevent hemorrhagic cystitis caused by ifosfamide and cyclophosphamide. Its oral bioavailability makes it an ideal candidate for oral administration, setting it apart from other chemotherapeutic agents that are not suitable for oral use.

**✓ Correct Answer: D. Mesna**