**Core Concept**
Lithium is a narrow therapeutic index medication used to treat bipolar disorder and major depressive disorder. Its plasma concentration must be closely monitored to avoid toxicity, which can be life-threatening. When co-administered with antihypertensive agents, certain drugs can increase lithium levels, leading to toxicity.

**Why the Correct Answer is Right**
Lithium toxicity is often caused by non-steroidal anti-inflammatory drugs (NSAIDs), which increase renal lithium clearance, leading to increased lithium levels in the blood. However, in this case, we are looking for an antihypertensive agent that should be avoided. The correct answer is **C. Spironolactone**. Spironolactone is a potassium-sparing diuretic that can increase lithium levels by competing with lithium for renal excretion. This can lead to lithium toxicity, particularly in patients with pre-existing kidney disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** Atenolol is a beta-blocker that is not known to interact with lithium and increase the risk of toxicity.
* **Option B:** Enalapril, an ACE inhibitor, is not typically associated with increased lithium levels and can be used cautiously in patients with lithium therapy.
* **Option D:** Amlodipine, a calcium channel blocker, does not have a known interaction with lithium that would increase the risk of toxicity.

**Clinical Pearl / High-Yield Fact**
When managing patients on lithium therapy, it is essential to monitor renal function, as lithium clearance is reduced in patients with kidney disease. This can lead to lithium toxicity, even at therapeutic doses.

**Correct Answer:** C. Spironolactone.