**Core Concept**
Electroconvulsive Therapy (ECT) is a psychiatric treatment that involves the use of electrical impulses to induce seizures in patients. ECT is typically used to treat severe, treatment-resistant depression, mania, and catatonia. The therapeutic effects of ECT are thought to be related to the modulation of neurotransmitter systems, particularly the increase in dopamine and norepinephrine release.

**Why the Correct Answer is Right**
ECT is a highly effective treatment for severe, treatment-resistant depression, particularly in patients with a high risk of suicide. It is also used to treat mania and catatonia, which are associated with a high risk of morbidity and mortality. The mechanism of action of ECT involves the induction of a generalized seizure, which leads to an increase in the release of neurotransmitters such as dopamine and norepinephrine. This increase in neurotransmitter release is thought to be responsible for the therapeutic effects of ECT.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because ECT is not typically used as a first-line treatment for schizophrenia. While ECT may be used as an adjunctive treatment for schizophrenia, particularly in patients with treatment-resistant symptoms, it is not a primary treatment for this condition.

**Option B:** Incorrect because ECT is not typically used as a treatment for obsessive-compulsive disorder (OCD). While ECT may be used as an adjunctive treatment for OCD, particularly in patients with treatment-resistant symptoms, it is not a primary treatment for this condition.

**Option C:** Incorrect because ECT is not typically used as a treatment for substance use disorders. While ECT may be used as an adjunctive treatment for patients with co-occurring psychiatric and substance use disorders, it is not a primary treatment for substance use disorders.

**Clinical Pearl / High-Yield Fact**
It's worth noting that ECT is a highly effective treatment for severe, treatment-resistant depression, particularly in patients with a high risk of suicide. However, ECT should only be used under the close supervision of a qualified psychiatrist, and patients should be carefully evaluated for potential side effects and contraindications.

**Correct Answer: C. ECT is currently indicated as a line of treatment in the following conditions except: Substance use disorders.**