**Core Concept**
Prolonged QT interval is a significant concern in electroconvulsive therapy (ECT), as it increases the risk of torsades de pointes, a potentially life-threatening arrhythmia. ECT can lead to a transient prolongation of the QT interval, but a pre-existing prolonged QT interval can increase this risk.

**Why the Correct Answer is Right**
A prolonged QT interval is a known contraindication to ECT due to the increased risk of developing torsades de pointes. This is because the prolonged QT interval can increase the duration of the action potential in the heart, making it more susceptible to arrhythmias. In ECT, the electrical stimulus can further prolong the QT interval, increasing the risk of torsades de pointes.

**Why Each Wrong Option is Incorrect**
**Option A:** **Pregnancy** is not a contraindication to ECT, as it is a safe and effective treatment for severe depression, especially in pregnant women. In fact, ECT is often considered a first-line treatment in pregnant women with severe depression.

**Option B:** **History of head trauma** is not a contraindication to ECT, as it does not increase the risk of further brain injury. However, it is essential to carefully evaluate the patient's medical history and consider any potential risks before administering ECT.

**Option C:** **Severe hypertension** is not a contraindication to ECT, as it can be managed with appropriate medication and monitoring. In fact, ECT can be a useful treatment for patients with severe depression and hypertension.

**Clinical Pearl / High-Yield Fact**
ECT is a safe and effective treatment for severe depression, especially in patients who have not responded to other treatments. It is essential to carefully evaluate each patient's medical history and consider any potential risks before administering ECT.

**Correct Answer: A. Prolonged QT interval.**