**Core Concept**
Atropine is a muscarinic receptor antagonist that blocks the action of acetylcholine in the parasympathetic nervous system. The tissues most sensitive to atropine are those that have a high concentration of muscarinic receptors, particularly in the salivary, bronchial, and sweat glands.

**Why the Correct Answer is Right**
These glands are part of the parasympathetic nervous system's target organs, which are regulated by the release of acetylcholine from postganglionic neurons. Atropine binds to muscarinic receptors on these glands, preventing acetylcholine from stimulating the glands to produce saliva, mucus, or sweat. This results in decreased secretions from these glands. Additionally, atropine also affects the smooth muscle in the gastrointestinal tract and bronchial tubes, but its effect is not as pronounced as on the salivary, bronchial, and sweat glands.

**Why Each Wrong Option is Incorrect**
**Option B:** The gastric parietal cells are sensitive to histamine and acetylcholine, which stimulate gastric acid secretion. Atropine does reduce gastric acid secretion, but its effect is not as pronounced as on the salivary, bronchial, and sweat glands.
**Option C:** Smooth muscle and autonomic effectors are indeed affected by atropine, but the effect is not as marked as on the salivary, bronchial, and sweat glands. Additionally, atropine can also affect the heart, particularly in high doses, but this is not the most sensitive tissue to atropine.

**Clinical Pearl / High-Yield Fact**
Remember that the salivary, bronchial, and sweat glands are the most sensitive tissues to atropine due to their high concentration of muscarinic receptors. Knowing this will help you predict the effects of atropine on these glands and avoid potential pitfalls in clinical scenarios.

**Correct Answer:**
✓ Correct Answer: A. The salivary, bronchial and sweat glands