**Core Concept**
Hypermagnesemia is a condition characterized by elevated serum magnesium levels, which can lead to various systemic effects due to its role in neuromuscular and cardiovascular functions. Magnesium plays a crucial role in the regulation of neuromuscular transmission, cardiac rhythm, and vascular tone.

**Why the Correct Answer is Right**
Hypermagnesemia can cause neuromuscular blockade, leading to muscle weakness, flaccidity, and respiratory depression. The elevated magnesium levels can also affect cardiac function, causing bradycardia, heart block, and eventually cardiac arrest. Additionally, hypermagnesemia can lead to hypotension due to vasodilation and decreased cardiac contractility.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because muscle weakness is indeed a feature of hypermagnesemia. However, this option is too vague, and other options might be more distinctive.

**Option B:** Incorrect because respiratory depression is a known consequence of hypermagnesemia. However, this option is too general and does not provide a distinct feature.

**Option C:** Incorrect because this option is not a feature of hypermagnesemia. Hypermagnesemia typically causes hypotension due to vasodilation and decreased cardiac contractility, not hypertension.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of hypermagnesemia is the presence of a "saddlebag" shape on the ECG, characterized by a prolonged PR interval and a widened QRS complex.

**Correct Answer:** C. Hypertension