**Core Concept**
Fexofenadine is a non-sedating antihistamine used to treat allergic reactions. It belongs to the class of drugs known as selective histamine H1 receptor antagonists. The question asks about its metabolic product, which involves understanding the pharmacokinetics of the drug.

**Why the Correct Answer is Right**
Fexofenadine is a metabolite of terfenadine, a first-generation antihistamine that was withdrawn from the market due to its potential to cause cardiac arrhythmias. Terfenadine undergoes extensive first-pass metabolism in the liver, primarily by the enzyme CYP3A4, to form fexofenadine. Fexofenadine is then excreted in the urine. The selective metabolism of terfenadine to fexofenadine reduces the risk of cardiac side effects associated with the parent compound.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the metabolism of fexofenadine. Loratadine is another non-sedating antihistamine, but it is not a precursor to fexofenadine.
**Option B:** This option is incorrect because it does not relate to the metabolism of fexofenadine. Cetirizine is a second-generation antihistamine, but it is not a metabolite of terfenadine.
**Option D:** This option is incorrect because it does not accurately describe the metabolism of fexofenadine. Desloratadine is a metabolite of loratadine, but not fexofenadine.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the metabolism of fexofenadine involves the enzyme CYP3A4, which can be inhibited by certain drugs, such as ketoconazole. This interaction can increase the levels of fexofenadine and potentially lead to adverse effects.

**Correct Answer: C. Terfenadine.**