**Core Concept**
Clonidine, a centrally acting alpha-2 agonist, is used to treat hypertension. Its antihypertensive action is mediated through the activation of alpha-2 adrenergic receptors in the brain, which decreases sympathetic outflow and leads to vasodilation.

**Why the Correct Answer is Right**
The tricyclic antidepressant (TCA) prescribed likely contains a strong noradrenergic reuptake inhibitor, such as imipramine. This would increase the levels of norepinephrine in the synaptic cleft, which in turn would block the alpha-2 receptors activated by clonidine. As a result, the antihypertensive action of clonidine is abolished. Enalapril, a converting enzyme inhibitor, and diltiazem, a calcium channel blocker, do not have their antihypertensive action affected by TCAs in this manner. Atenolol, a beta-1 selective blocker, does not have its action abolished by TCAs.

**Why Each Wrong Option is Incorrect**
**Option A:** Enalapril's antihypertensive action is not affected by TCAs, as it works through a different mechanism, inhibiting the conversion of angiotensin I to angiotensin II.

**Option C:** Atenolol's action is not abolished by TCAs, as it works through a different mechanism, blocking beta-1 receptors in the heart, which does not involve the noradrenergic system.

**Option D:** Diltiazem's action is not abolished by TCAs, as it works through a different mechanism, blocking calcium channels in the heart and blood vessels, which does not involve the noradrenergic system.

**Clinical Pearl / High-Yield Fact**
When prescribing a centrally acting antihypertensive agent like clonidine, it's essential to be aware of potential interactions with medications that affect the noradrenergic system, such as TCAs.

**✓ Correct Answer: B. Clonidine**