**Core Concept**
Cyclophosphamide is a chemotherapeutic agent used in the treatment of various cancers and autoimmune disorders. It is a nitrogen mustard alkylating agent that undergoes metabolic activation in the liver, which is then excreted primarily through the kidneys.

**Why the Correct Answer is Right**
Cyclophosphamide is metabolized in the liver by the enzyme cytochrome P450 to form 4-hydroxycyclophosphamide, which is then converted to phosphoramide mustard and acrolein. The majority of the drug is excreted in the urine as the parent compound, with a smaller fraction excreted as its metabolites. The kidneys play a crucial role in eliminating cyclophosphamide and its metabolites from the body.

**Why Each Wrong Option is Incorrect**
**Option A:** The lungs are not primarily involved in the excretion of cyclophosphamide.
**Option B:** While the liver metabolizes cyclophosphamide, it is not the primary route of excretion for this drug.
**Option D:** The skin is not a significant route of excretion for cyclophosphamide.

**Clinical Pearl / High-Yield Fact**
It is essential to note that cyclophosphamide can cause hemorrhagic cystitis due to the formation of acrolein, a toxic metabolite that damages the bladder mucosa. Adequate hydration and urinary alkalinization can help prevent this complication.

**✓ Correct Answer: C. Kidney. The kidneys are the primary route of excretion for cyclophosphamide.**