**Core Concept**
Severe cardiovascular disease is a significant concern when considering electroconvulsive therapy (ECT) due to the potential for cardiac complications during and after treatment. ECT can cause an increase in heart rate, blood pressure, and cardiac output, which may be problematic for patients with pre-existing cardiovascular conditions.

**Why the Correct Answer is Right**
Patients with severe cardiovascular disease, such as unstable angina, recent myocardial infarction, or severe hypertension, are at a higher risk of cardiac complications during ECT. The increased sympathetic activity and potential for arrhythmias make ECT a relatively contraindicated treatment in these patients. **Option C: Unstable angina** is a correct relative contraindication for ECT due to the risk of cardiac complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Recent head injury may be a contraindication for ECT due to the risk of worsening intracranial pressure, but it is not the best answer in this context.
**Option B:** Pregnancy is not typically a contraindication for ECT, as its benefits can outweigh the risks in severe cases of depression.

**Clinical Pearl / High-Yield Fact**
ECT is generally considered safe in patients with mild to moderate cardiovascular disease, but close monitoring and careful selection of patients are essential.

**Correct Answer:** C. Unstable angina