**Core Concept**
Atropine is an anticholinergic medication that blocks the action of acetylcholine at muscarinic receptors in the parasympathetic nervous system. The duration of its effect varies among different organs due to differences in acetylcholinesterase enzyme activity and receptor density.

**Why the Correct Answer is Right**
The longest duration of atropine's effect is seen in the heart, where it can last for up to 24 hours. This is because the heart has a low concentration of acetylcholinesterase, which is the enzyme responsible for breaking down acetylcholine. As a result, the remaining acetylcholine is available for a longer period, and atropine's blocking effect is sustained. Additionally, the heart's muscarinic receptors have a longer half-life, contributing to the prolonged effect of atropine.

**Why Each Wrong Option is Incorrect**
**Option A:** In the gut, the effect of atropine is shorter due to the high concentration of acetylcholinesterase, which rapidly breaks down acetylcholine.
**Option B:** In the eye, the effect of atropine is intermediate in duration, as the eye has a moderate concentration of acetylcholinesterase and muscarinic receptors.
**Option C:** In the bladder, the effect of atropine is also intermediate in duration, as the bladder has a moderate concentration of acetylcholinesterase and muscarinic receptors.

**Clinical Pearl / High-Yield Fact**
Remember that the duration of atropine's effect is inversely related to the concentration of acetylcholinesterase in the organ. This is a useful fact to keep in mind when considering the pharmacodynamics of anticholinergic medications.

**Correct Answer: C. Eye**