**Core Concept**
The H1 receptors are a subtype of histamine receptors found primarily in the brain, smooth muscle, and endothelium. Histamine is a neurotransmitter and a potent vasodilator that plays a crucial role in allergic responses, inflammation, and immune reactions. Drugs that act on H1 receptors are primarily used to treat allergic conditions, such as allergic rhinitis, urticaria, and anaphylaxis.

**Why the Correct Answer is Right**
The correct answer is the drug that does not have an active metabolite. Active metabolites are metabolites that retain the pharmacological activity of the parent compound. The drugs listed in the options are used to treat allergic conditions by blocking the action of histamine on H1 receptors.

**Option A:** **Diphenhydramine** has active metabolites, including 2-pyridylmethyl alcohol and 2-pyridylacetic acid, which contribute to its prolonged sedative effects.
**Option B:** **Chlorpheniramine** also has active metabolites, including 2-pyridylmethyl alcohol and 2-pyridylacetic acid, which are responsible for its sedative effects.
**Option C:** **Cetirizine** has a relatively long half-life and does not have significant active metabolites, making it a popular choice for treating allergic conditions.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the duration of action and side effects of H1 receptor antagonists can be influenced by their active metabolites. This is particularly important when choosing a medication for patients with renal or hepatic impairment, as the metabolism of these drugs may be altered.

**Correct Answer: C. Cetirizine**