**Core Concept**
Muscarinic antagonists, also known as anticholinergics, are a class of medications that block the action of acetylcholine at muscarinic receptors in the central and peripheral nervous system. These receptors are part of the parasympathetic nervous system, which promotes rest and digestion.

**Why the Correct Answer is Right**
Muscarinic antagonists work by competitively inhibiting the binding of acetylcholine to muscarinic receptors, thereby reducing the parasympathetic tone. This leads to a range of effects, including:
* Decreased salivation and sweating
* Increased heart rate and cardiac output
* Relaxed smooth muscle in the gastrointestinal tract, bladder, and bronchi
* Pupillary dilation

**Why Each Wrong Option is Incorrect**
* **Option A:** Muscarinic antagonists actually decrease salivation, not increase it. The correct effect is due to the inhibition of muscarinic receptors in the salivary glands.
* **Option B:** Muscarinic antagonists actually increase heart rate, not decrease it. The blockade of muscarinic receptors in the sinoatrial node leads to an increase in heart rate.
* **Option C:** Muscarinic antagonists actually relax smooth muscle in the gastrointestinal tract, not contract it. The blockade of muscarinic receptors in the gastrointestinal tract leads to a decrease in peristalsis and increased gastric emptying time.

**Clinical Pearl / High-Yield Fact**
Muscarinic antagonists can cause anticholinergic side effects such as dry mouth, blurred vision, and urinary retention. These effects are due to the blockade of muscarinic receptors in various parts of the body.

**Correct Answer:** A.