**Core Concept**
Atropine is a non-selective muscarinic receptor antagonist that inhibits the action of acetylcholine in the central and peripheral nervous system. It is used to treat various conditions, including bradycardia, bronchospasm, and excessive salivation.

**Why the Correct Answer is Right**
Atropine works by blocking the action of acetylcholine on muscarinic receptors in the heart, leading to an increase in heart rate. It also relaxes smooth muscles in the airways, causing bronchodilation. Additionally, atropine can reduce secretions by blocking the parasympathetic stimulation of salivary and sweat glands. The mechanism of action of atropine is mediated through the inhibition of the enzyme acetylcholinesterase, which breaks down acetylcholine.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as atropine is indeed used to treat bradycardia, which is a heart rate that is slower than normal.
**Option B:** Atropine is also used to treat bronchospasm associated with asthma and chronic obstructive pulmonary disease (COPD), making this option incorrect.
**Option C:** Excessive salivation is another condition that atropine is used to treat, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
Atropine can cause anticholinergic side effects, such as dry mouth, blurred vision, and constipation, due to its blockade of muscarinic receptors in the body.

**Correct Answer: D.**