**Core Concept:** QT prolongation refers to an abnormal increase in the duration of the QT interval on an electrocardiogram (ECG), which is associated with an increased risk of life-threatening arrhythmias, such as torsades de pointes. QT interval represents the time it takes for the heart to recharge between beats, particularly the duration of the T wave, which is influenced by the balance between the rate of potassium and sodium ions repolarization in cardiac myocytes.

**Why the Correct Answer is Right:** QT prolongation can be caused by various factors, including:

1. Drug-induced QT prolongation (Correct Answer): Certain medications can interfere with potassium and sodium ion repolarization, leading to an extended QT interval. This is often due to the blockage or reduction of the repolarizing potassium currents (e.g., IKr, IKs), which increases the duration of the QT interval and increases the risk of arrhythmias.

2. **Option A (Blockade of IKs):** Inhibition of the delayed rectifier potassium current (IKs) prolongs the QT interval as it slows potassium repolarization. This is seen with drugs like ranolazine, dofetilide, and sotalol.

3. **Option B (Blockade of IKr):** Blockade of the rapid delayed rectifier potassium current (IKr) leads to QT prolongation, as it reduces the repolarizing potassium currents. This is seen with drugs like dofetilide, cisapride, and ivabradine.

4. **Option C (Increased sympathetic activity):** Elevated sympathetic activity, as in catecholaminergic storms or severe sepsis, can lead to QT prolongation. This is due to increased heart rate and decreased heart rate variability, which prolongs the QT interval.

5. **Option D (Low potassium):** Hypokalaemia can cause QT prolongation as potassium plays a role in repolarization of cardiac cells. Low potassium levels affect the balance between repolarizing potassium currents and depolarizing sodium currents, leading to QT prolongation.

**Why Each Wrong Option is Incorrect:**

6. **Option A (Increased sympathetic activity):** While increased sympathetic activity can lead to tachycardia, which may indirectly cause QT prolongation, it is incorrect because the primary cause of QT prolongation in this case is the result of increased heart rate, not direct interference with potassium or sodium currents.

7. **Option B (Low potassium):** Although hypokalaemia can lead to QT prolongation, it is incorrect because the primary cause of QT prolongation in this case is the direct interference with potassium currents, specifically IK1 and IKs, rather than a hypokalaemia-related effect on the QT interval.

8. **Option D (Decreased magnesium):** Magnesium is a cofactor in the action potential duration, but its deficiency does not directly prolong the QT interval. Hypomagnesaemia can lead to cardiac arrhythmias, but it is incorrect for this question because the primary