**Core Concept**
Anticholinergic properties refer to a drug’s ability to block acetylcholine at muscarinic receptors. Among antihistamines, this effect is most prominent in first-generation agents that cross the blood-brain barrier and have significant central nervous system activity.

**Why the Correct Answer is Right**
Diphenhydramine is a first-generation antihistamine with high anticholinergic activity due to its strong affinity for muscarinic receptors. It readily crosses the blood-brain barrier and blocks both H1 receptors and muscarinic receptors, leading to sedation, dry mouth, blurred vision, and urinary retention. This property is a hallmark of older antihistamines and is clinically significant in older adults.

**Why Each Wrong Option is Incorrect**
Option A: Astemizole has low anticholinergic activity and was withdrawn due to cardiac toxicity, not anticholinergic effects.
Option C: Terfenadine is a second-generation antihistamine with minimal anticholinergic effects and no significant central penetration.
Option D: Fexofenadine is a second-generation antihistamine with negligible anticholinergic properties and no sedative effects.

**Clinical Pearl / High-Yield Fact**
First-generation antihistamines like diphenhydramine have high anticholinergic effects and are associated with cognitive impairment in elderly patients — a key safety concern in geriatric use.

✓ Correct Answer: B. Diphenhydramine