**Core Concept:** Alpha adrenergic agonists are a class of medications that bind to and activate alpha-adrenergic receptors, leading to various physiological effects including vasoconstriction, increased heart rate, and elevated blood pressure. These effects contribute to the antihistamine and decongestant properties of alpha agonist-containing nasal decongestants.

**Why the Correct Answer is Right:** The earliest sign to appear following the ingestion of an alpha adrenergic agonist is vasoconstriction in the mucosal blood vessels of the nasal passages. This results in reduced blood flow to the nasal mucosa and decreased congestion, leading to relief of nasal symptoms.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** Vasodilation - This is the opposite of vasoconstriction and would be the last sign to appear due to ingesting an alpha agonist.

B. **Option B:** Increased heart rate - While this may occur, the focus of the question is on the earliest sign, which is vasoconstriction.

C. **Option C:** Elevated blood pressure - Similar to increased heart rate, while this might occur, the primary concern is the onset of the first symptom, which is vasoconstriction.

D. **Option D:** Cardiac arrhythmias - This is a late complication and not the earliest sign observed after alpha agonist ingestion.

**Clinical Pearl:** Vasoconstriction is the most relevant physiological effect of alpha adrenergic agonists and is the earliest sign to appear after exposure, making option C (elevated blood pressure) the correct answer.

**Correct Answer:** C. Elevated blood pressure (hypertension) - Following the ingestion of an alpha agonist, vasoconstriction occurs first, which leads to the reduction of blood flow to the nasal passages, resulting in decreased congestion and relief of nasal symptoms. While hypertension (elevated blood pressure) may occur, it is not the earliest sign and is not directly related to the primary effect of alpha agonists, which is vasoconstriction.