**Core Concept**
Heptahelical receptors, also known as G-protein-coupled receptors (GPCRs), are a class of transmembrane receptors that play a crucial role in signal transduction pathways. These receptors have a characteristic seven-transmembrane domain structure and are involved in various physiological processes, including neurotransmission, hormone regulation, and sensory perception.

**Why the Correct Answer is Right**
Drugs that act through heptahelical receptors bind to the extracellular domain of the receptor, triggering a conformational change that activates or inhibits the associated G-protein. This leads to the activation or inhibition of downstream signaling pathways, resulting in various physiological effects. Examples of heptahelical receptors include adrenergic receptors (e.g., alpha-1 and beta-2), muscarinic acetylcholine receptors, and serotonin receptors.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a specific drug that acts through heptahelical receptors.
**Option B:** This option is incorrect because it is not a correct example of a drug that acts through heptahelical receptors.
**Option C:** This option is incorrect because it is not a correct example of a drug that acts through heptahelical receptors.

**Clinical Pearl / High-Yield Fact**
Heptahelical receptors are the largest family of membrane receptors and are involved in a wide range of physiological processes. They are also a major target for many therapeutic drugs, including beta blockers, antihistamines, and antipsychotics.

**Correct Answer: C. Dopamine D2 receptor antagonist. Haloperidol**