**Core Concept**
Sumatriptan is a selective serotonin receptor agonist, specifically targeting the 5-HT1D and 5-HT1B receptors. This action is crucial for its antimigraine effects.

**Why the Correct Answer is Right**
Sumatriptan's mechanism of action involves the activation of the 5-HT1D and 5-HT1B receptors, which are found on the terminals of the trigeminal nerve. By stimulating these receptors, sumatriptan causes vasoconstriction of the intracranial blood vessels and inhibits the release of pro-inflammatory neuropeptides, thereby reducing migraine headache severity. This selective agonism is distinct from the effects of other serotonin receptor subtypes.

**Why Each Wrong Option is Incorrect**
* **Option B:** 5-HT2 receptors are not the primary target of sumatriptan, as their activation would increase the release of pro-inflammatory neuropeptides, exacerbating migraine symptoms.
* **Option C:** 5-HT3 receptors are primarily involved in the regulation of emesis and are not directly implicated in the antimigraine action of sumatriptan.
* **Option D:** 5-HT4 receptors are involved in modulating gut motility and secretion, and are not relevant to the mechanism of action of sumatriptan.

**Clinical Pearl / High-Yield Fact**
The 5-HT1B receptor is the primary target for the antimigraine action of sumatriptan, while the 5-HT1D receptor is also involved. This dual action is responsible for the efficacy of sumatriptan in treating migraine headaches.

**Correct Answer:**
✓ Correct Answer: A. 5HT1D/1B