**Core Concept**
Torsade-de-pointes is a specific type of polymorphic ventricular tachycardia characterized by a long QT interval on the electrocardiogram. It is a potentially life-threatening arrhythmia that can occur in response to various factors, including electrolyte imbalances.

**Why the Correct Answer is Right**
Hypomagnesemia, or low magnesium levels, is a well-established cause of torsade-de-pointes. Magnesium plays a crucial role in stabilizing the cardiac cell membrane and regulating potassium channels. In hypomagnesemia, the reduced magnesium levels can lead to an increase in potassium permeability, causing prolongation of the QT interval and increasing the risk of torsade-de-pointes. This is often seen in patients with magnesium deficiency, particularly those taking certain medications such as diuretics.

**Why Each Wrong Option is Incorrect**
* **Option A:** Hypermagnesemia, or high magnesium levels, is not typically associated with torsade-de-pointes. In fact, high magnesium levels can actually help to stabilize the cardiac cell membrane and reduce the risk of arrhythmias.
* **Option B:** Metabolic acidosis is a condition characterized by an excess of acid in the blood, but it is not a direct cause of torsade-de-pointes. While metabolic acidosis can lead to various cardiac complications, it is not specifically linked to the development of this arrhythmia.
* **Option D:** Metabolic alkalosis is a condition characterized by an excess of base in the blood, and it is not typically associated with torsade-de-pointes. In fact, metabolic alkalosis can sometimes help to reduce the risk of arrhythmias by stabilizing the cardiac cell membrane.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that hypomagnesemia can often be overlooked in patients with torsade-de-pointes, as it may not always be apparent on initial laboratory tests. A low magnesium level should always be considered in the differential diagnosis of this arrhythmia.

✓ Correct Answer: C. Hypomagnesemia