**Core Concept**
Dialectical behaviour therapy (DBT) is a type of psychotherapy that focuses on developing emotional regulation, distress tolerance, and interpersonal skills. It was primarily designed for individuals with borderline personality disorder (BPD) who exhibit intense emotional dysregulation and self-destructive behaviors.

**Why the Correct Answer is Right**
DBT was developed by Marsha Linehan to specifically address the complex emotional and behavioral challenges faced by individuals with BPD. The therapy combines elements of cognitive-behavioral therapy (CBT) with mindfulness techniques and emphasizes the development of a "wise mind" that integrates rational thinking and emotional experience. By teaching individuals with BPD skills to manage their emotions, tolerate distress, and maintain healthy relationships, DBT has been shown to be effective in reducing suicidal behaviors, self-injury, and other impulsive behaviors associated with BPD.

**Why Each Wrong Option is Incorrect**
**Option A:** DBT is not primarily used in the management of post-traumatic stress disorder (PTSD), although some of its techniques may be useful in treating PTSD symptoms.
**Option B:** DBT is not typically used for treating obsessive-compulsive disorder (OCD), as its focus is on emotional regulation and interpersonal skills rather than the specific symptoms of OCD.
**Option C:** DBT is not a first-line treatment for major depressive disorder (MDD), although it may be used as an adjunctive therapy for individuals with MDD who also have BPD or other co-occurring disorders.

**Clinical Pearl / High-Yield Fact**
Remember that DBT is a skills-based therapy that focuses on teaching individuals with BPD specific skills to manage their emotions, tolerate distress, and maintain healthy relationships. It is not a "talk therapy" that focuses on exploring past experiences or traumas.

**Correct Answer:** C. Borderline personality disorder.