**Core Concept**
Electroconvulsive therapy (ECT) is a psychiatric treatment that involves the deliberate induction of a controlled seizure by passing an electric current through the brain. ECT is used to treat severe, treatment-resistant depression, particularly when there is a high risk of suicide.

**Why the Correct Answer is Right**
ECT is the most effective treatment for severe depression with suicidal tendencies due to its rapid onset of action and ability to produce significant improvements in mood and cognition. ECT works by altering the brain's neurotransmitter levels, particularly serotonin, dopamine, and norepinephrine, which are involved in mood regulation. The therapeutic effects of ECT are thought to be mediated by the release of neurotrophic factors, which promote neuronal growth and survival. In the context of suicidal depression, ECT can help to rapidly reduce symptoms and prevent further episodes of suicidal behavior.

**Why Each Wrong Option is Incorrect**
**Option A:** Clozapine is an atypical antipsychotic medication used primarily to treat schizophrenia. While it can be used off-label for treatment-resistant depression, it is not the first-line treatment for depression with suicidal tendencies.

**Option B:** Mirtazapine is an antidepressant medication that works by increasing the activity of certain neurotransmitters, such as serotonin and norepinephrine. However, it is not as effective as ECT for severe depression with suicidal tendencies.

**Option D:** Olanzapine is another atypical antipsychotic medication that can be used off-label for treatment-resistant depression. However, it is not the preferred treatment for depression with suicidal tendencies due to its potential for weight gain and metabolic side effects.

**Clinical Pearl / High-Yield Fact**
When treating depression with suicidal tendencies, it is essential to consider the patient's immediate safety and to initiate treatment with a medication or intervention that can provide rapid symptom relief, such as ECT. In addition to ECT, other high-yield treatments for suicidal depression include medications like ketamine and the use of a "safety plan" to help patients identify and cope with suicidal thoughts.

**✓ Correct Answer: C. ECT**