**Core Concept**
The combination of spironolactone and enalapril is contraindicated due to the risk of hyperkalemia, a condition characterized by elevated serum potassium levels. This is a classic example of a pharmacodynamic interaction between two medications.

**Why the Correct Answer is Right**
When spironolactone is used concurrently with enalapril, a potassium-sparing diuretic, the risk of hyperkalemia increases significantly. Spironolactone acts as an aldosterone antagonist, which leads to increased potassium retention in the body. Enalapril, an ACE inhibitor, also causes potassium retention by reducing aldosterone levels and decreasing sodium reabsorption in the kidneys. The concurrent use of these medications can lead to a dangerous accumulation of potassium in the blood.

**Why Each Wrong Option is Incorrect**

* **Option A:** Hyperglycemia - This is incorrect because spironolactone and enalapril do not have a direct effect on blood glucose levels.
* **Option B:** Hypotension - While both medications can cause hypotension, this is not the primary concern with their combination.
* **Option C:** Increased risk of renal failure - This is incorrect because neither medication is primarily associated with renal failure.

**Clinical Pearl / High-Yield Fact**
When prescribing diuretics, always consider potential interactions with other medications that can affect potassium levels, such as ACE inhibitors or potassium-sparing diuretics.

**Correct Answer: C. Hyperkalemia**