**Core Concept**
The question is testing the student's knowledge of the pharmacological profiles of different antihistamines, specifically their effects on the cholinergic system. Antihistamines can have varying degrees of anticholinergic activity, which can lead to side effects such as dry mouth, urinary retention, and constipation.

**Why the Correct Answer is Right**
Fexofenadine is a second-generation non-sedating antihistamine that has a very low incidence of cholinergic side effects. This is because it does not readily cross the blood-brain barrier and has minimal anticholinergic activity. Fexofenadine works by selectively inhibiting the peripheral H1 receptors, which are responsible for the symptoms of allergic rhinitis and urticaria.

**Why Each Wrong Option is Incorrect**
**Option A:** Diphenhydramine is a first-generation antihistamine that has significant anticholinergic activity, leading to side effects such as dry mouth, drowsiness, and urinary retention. Its cholinergic side effects are a major drawback in its use.

**Option B:** Loratadine is another second-generation antihistamine, but it has a higher incidence of cholinergic side effects compared to fexofenadine. While it is generally well-tolerated, it can cause dry mouth and other anticholinergic side effects in some patients.

**Option C:** Cetirizine is a third-generation antihistamine that has a higher incidence of cholinergic side effects compared to fexofenadine. It can cause dry mouth, urinary retention, and constipation in some patients, making it a less favorable option for those with a history of cholinergic side effects.

**Clinical Pearl / High-Yield Fact**
When choosing an antihistamine, consider the patient's individual needs and medical history. Second-generation antihistamines like fexofenadine are generally preferred for their minimal anticholinergic activity and lower risk of side effects.

**Correct Answer:** C. Fexofenadine