**Core Concept**
The first dose phenomenon, also known as the 'first-dose effect,' refers to a sudden and significant drop in blood pressure following the initial administration of a particular medication. This phenomenon is particularly concerning in elderly patients due to their compromised cardiovascular system.

**Why the Correct Answer is Right**
Prazosin is an alpha-1 adrenergic receptor antagonist primarily used to treat hypertension and benign prostatic hyperplasia. Its mechanism of action involves blocking alpha-1 receptors, leading to vasodilation and a consequent decrease in blood pressure. In elderly patients, the first dose of prazosin can cause an exaggerated hypotensive effect due to decreased baroreceptor sensitivity and compensatory mechanisms. This can result in orthostatic hypotension, dizziness, and even syncope.

**Why Each Wrong Option is Incorrect**
**Option A:** Metformin is an oral antidiabetic drug that decreases hepatic glucose production and increases insulin sensitivity. It does not cause a first-dose phenomenon and is not associated with significant hypotension.

**Option C:** Acarbose is an alpha-glucosidase inhibitor used to manage type 2 diabetes mellitus. It works by slowing the digestion of carbohydrates, thereby reducing postprandial glucose peaks. Acarbose is not associated with the first-dose phenomenon.

**Option D:** Noradrenaline, also known as norepinephrine, is a potent vasoconstrictor used to treat severe hypotension and septic shock. Its administration would increase blood pressure, not decrease it, making it an incorrect choice for the first-dose phenomenon.

**Clinical Pearl / High-Yield Fact**
When initiating alpha-1 adrenergic receptor antagonists like prazosin, especially in elderly patients, it is crucial to monitor blood pressure closely and start with a low dose to avoid the first-dose phenomenon.

**✓ Correct Answer: B. Prazosin**