**Core Concept**
In organophosphate poisoning, atropinization is crucial to counteract the muscarinic effects of acetylcholine accumulation. The goal is to restore normal cholinergic balance by inhibiting acetylcholine at muscarinic receptors.

**Why the Correct Answer is Right**
The most reliable clinical endpoint for adequate atropinization is the resolution of miosis (pupil constriction). Atropine, a muscarinic antagonist, blocks acetylcholine's effects at muscarinic receptors in the eye, leading to pupil dilation. This indicator is specific and sensitive for atropine's muscarinic antagonism. Additionally, the resolution of miosis correlates with improved respiratory function and reduced muscle fasciculations.

**Why Each Wrong Option is Incorrect**
**Option A:** Heart rate is not a reliable indicator, as atropine can increase heart rate through vagal blockade, but this does not necessarily indicate adequate atropinization in organophosphate poisoning.

**Option B:** Gastric secretions may decrease with atropine, but this is not a specific or sensitive endpoint for adequate atropinization.

**Option C:** Muscle fasciculations may decrease with atropine, but this is not a reliable indicator, as muscle weakness and paralysis can persist despite adequate atropinization.

**Clinical Pearl / High-Yield Fact**
Remember, in organophosphate poisoning, atropinization is most effectively monitored by the resolution of miosis, as this reflects the blockade of acetylcholine's effects at muscarinic receptors.

**Correct Answer:** C. Resolution of miosis.