**Core Concept**
The management of severe hypertension in the elderly requires careful consideration of the underlying pathophysiology and potential drug interactions. In this context, the choice of antihypertensive agent can significantly impact patient outcomes.

**Why the Correct Answer is Right**
Prazosin, an alpha-1 adrenergic receptor antagonist, is not ideal for empirical use in severe hypertension due to its potential to cause a first-dose effect, characterized by a sudden and marked decrease in blood pressure. This can lead to orthostatic hypotension, dizziness, and even syncope, particularly in elderly patients who may have compromised cardiovascular reflexes. Furthermore, prazosin's mechanism of action can also increase the risk of cardiac failure in patients with underlying heart disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Enalapril, an angiotensin-converting enzyme (ACE) inhibitor, is a suitable choice for empirical use in severe hypertension, especially in elderly patients, as it has a relatively rapid onset of action and can be titrated to effect.
**Option B:** Amlodipine, a calcium channel blocker, is another effective option for managing severe hypertension, particularly in elderly patients, as it has a smooth and predictable onset of action.
**Option C:** Chlorthiazide, a thiazide diuretic, can be used empirically in severe hypertension, although caution is advised due to its potential to exacerbate hypovolemia and worsen renal function in certain patients.

**Clinical Pearl / High-Yield Fact**
In the elderly, initiation of antihypertensive therapy should be done cautiously, and patients should be closely monitored for signs of orthostatic hypotension, particularly in the first 24-48 hours after starting treatment. This is especially crucial for drugs like prazosin, which can have a more pronounced effect on blood pressure due to their mechanism of action.

**✓ Correct Answer: D. Prazosin**