**Core Concept**
The question is testing the understanding of antihistaminic poisoning, specifically the effects of a weak base antihistaminic drug on the body, particularly the central nervous system (CNS) and cardiovascular system (CVS).

**Why the Correct Answer is Right**
The antihistaminic drug in question is likely a first-generation antihistamine, which is a weak base capable of crossing the blood-brain barrier and entering the CNS. This can lead to central anticholinergic syndrome, characterized by symptoms such as tachycardia, hypertension, dry skin, and altered mental status. In this case, the child is experiencing tachycardia (heart rate 100/min) and normal blood pressure, but a normal respiratory rate is not typically associated with CNS effects.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because antihistamines typically cause dry skin and flushing, but hypotension is not a common presentation of antihistamine poisoning.

**Option B:** This option is incorrect because antihistamines can cause CNS effects, but mydriasis (dilated pupils) is not a characteristic feature of antihistamine poisoning.

**Option C:** This option is incorrect because antihistamines can cause CNS effects, but seizures are not typically associated with antihistamine poisoning in children.

**Clinical Pearl / High-Yield Fact**
In the context of antihistamine poisoning, it's essential to remember that first-generation antihistamines can cause central anticholinergic syndrome, characterized by tachycardia, dry skin, and altered mental status.

**Correct Answer:** C. **Seizures are not typically associated with antihistamine poisoning in children.**