**Core Concept**
Tolterodine is an antimuscarinic medication used to treat overactive bladder symptoms such as urgency, frequency, and incontinence. It works by blocking the action of acetylcholine at muscarinic receptors in the bladder muscle.

**Why the Correct Answer is Right**
Tolterodine acts as a competitive antagonist at muscarinic receptors, specifically M3 receptors, which are responsible for the contraction of the detrusor muscle in the bladder. By blocking M3 receptors, tolterodine reduces the frequency and urgency of urination, providing relief from overactive bladder symptoms. This mechanism is mediated through the inhibition of acetylcholine, a neurotransmitter that normally stimulates muscarinic receptors to contract the detrusor muscle.

**Why Each Wrong Option is Incorrect**
* **Option A:** Tolterodine does not act by blocking alpha-adrenergic receptors, which are involved in the contraction of smooth muscle in various organs, but not in the bladder.
* **Option B:** Tolterodine does not act by blocking 5-HT3 receptors, which are involved in the regulation of gastrointestinal motility and nausea.
* **Option C:** Tolterodine does not act by blocking beta-adrenergic receptors, which are involved in the regulation of heart rate and smooth muscle contraction in various organs.

**Clinical Pearl / High-Yield Fact**
Tolterodine and other antimuscarinic medications can have significant anticholinergic side effects, including dry mouth, constipation, and blurred vision. Patients on these medications should be monitored for these side effects and adjusted accordingly.

**Correct Answer: C. M3 receptors**