**Core Concept**
Overactive bladder is characterized by urgency incontinence, with or without urge incontinence, frequency, and nocturia. The primary goal of treatment is to increase bladder capacity and reduce symptoms.

**Why the Correct Answer is Right**
The correct answer is a muscarinic receptor antagonist, specifically an anticholinergic medication. These medications work by blocking the action of acetylcholine on the muscarinic receptors in the bladder detrusor muscle, leading to decreased bladder contractions and increased bladder capacity. This mechanism is particularly effective in treating overactive bladder symptoms, such as urgency and frequency.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is a direct-acting smooth muscle relaxant, which would not specifically target the muscarinic receptors in the bladder and may not provide sufficient relief for overactive bladder symptoms.

**Option B:** This option is incorrect because it is a selective serotonin reuptake inhibitor (SSRI), which is primarily used to treat depression and anxiety disorders. SSRIs are not typically used to treat overactive bladder.

**Option C:** This option is incorrect because it is an alpha-adrenergic receptor agonist, which would actually increase bladder outlet resistance and may worsen overactive bladder symptoms.

**Clinical Pearl / High-Yield Fact**
Anticholinergic medications, such as oxybutynin, are often used as first-line treatment for overactive bladder, but they can cause dry mouth, constipation, and urinary retention as side effects. It's essential to weigh the benefits and risks of these medications with each patient.

**Correct Answer:** C. Oxybutynin is a common anticholinergic medication used to treat overactive bladder.