**Core Concept**
The most significant adverse effect of ACE inhibition is related to the renin-angiotensin-aldosterone system (RAAS), which plays a crucial role in regulating blood pressure and fluid balance. ACE inhibitors work by blocking the conversion of angiotensin I to angiotensin II, leading to decreased vasoconstriction and aldosterone-mediated sodium retention.

**Why the Correct Answer is Right**
The correct answer is **hyperkalemia**. ACE inhibitors decrease the aldosterone-mediated sodium retention, leading to increased potassium levels. This is because aldosterone promotes potassium excretion in the urine. Additionally, ACE inhibitors can also decrease the conversion of angiotensin I to angiotensin II, which is a potent vasoconstrictor that stimulates the release of aldosterone. The decreased vasoconstriction and aldosterone levels lead to increased potassium levels.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypercalcemia is not a significant adverse effect of ACE inhibition. ACE inhibitors do not have a direct effect on calcium levels.

**Option B:** Hypotension is a common side effect of ACE inhibitors, but it is not the most significant adverse effect. ACE inhibitors can cause hypotension due to decreased vasoconstriction, but this can be managed with dose adjustments or combination therapy.

**Option C:** Renal failure is a potential adverse effect of ACE inhibitors, particularly in patients with pre-existing renal disease. However, it is not the most significant adverse effect.

**Clinical Pearl / High-Yield Fact**
It's essential to monitor potassium levels and renal function in patients taking ACE inhibitors, especially in those with pre-existing renal disease or cardiac failure.

**Correct Answer:** D. Hyperkalemia