**Core Concept**
Hyoscine, also known as scopolamine, is an anticholinergic medication that blocks the action of acetylcholine at muscarinic receptors in the central and peripheral nervous system. This action leads to various effects on the body, including reduced salivation, decreased bowel motility, and dilation of pupils.

**Why the Correct Answer is Right**
Hyoscine acts as a muscarinic receptor antagonist by binding to the receptor and preventing the action of acetylcholine, a neurotransmitter that stimulates these receptors. This results in a decrease in parasympathetic tone, leading to symptoms such as dry mouth, constipation, and urinary retention. Muscarinic receptors are a subtype of acetylcholine receptors, and hyoscine's action on these receptors is responsible for its anticholinergic effects.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because hyoscine is not primarily a nicotinic receptor antagonist, which would affect the nervous system in a different way.

**Option B:** This option is incorrect because hyoscine does not primarily act as an adrenergic receptor antagonist, which would affect the sympathetic nervous system.

**Option C:** This option is incorrect because hyoscine does not primarily act as a histamine receptor antagonist, which would affect allergic reactions and other processes.

**Clinical Pearl / High-Yield Fact**
Remember that anticholinergic medications like hyoscine can cause anticholinergic toxicity, which can manifest as confusion, hallucinations, and tachycardia, especially in the elderly or in patients with pre-existing medical conditions.

**Correct Answer: D. Muscarinic**