**Core Concept**
The most common cause of suicide is a complex interplay of various psychological, social, and biological factors. However, studies have consistently shown that mental health disorders, particularly depression, are a significant underlying cause of suicidal behavior.

**Why the Correct Answer is Right**
Research has shown that individuals with major depressive disorder (MDD) are at an increased risk of suicidal ideation and behavior. This is often due to the presence of depressive symptoms such as hopelessness, worthlessness, and anxiety, which can impair an individual's ability to cope with stress and lead to suicidal thoughts. The hypothalamic-pituitary-adrenal (HPA) axis is also dysregulated in individuals with depression, leading to increased levels of cortisol, which can further contribute to suicidal behavior.

**Why Each Wrong Option is Incorrect**
**Option A:** While substance abuse is a significant risk factor for suicidal behavior, it is not the most common cause of suicide.
**Option B:** Chronic pain is a significant risk factor for suicidal behavior, but it is not the most common cause of suicide.
**Option C:** Traumatic brain injury is a risk factor for suicidal behavior, but it is not the most common cause of suicide.

**Clinical Pearl / High-Yield Fact**
Depression is a treatable condition, and prompt recognition and treatment can significantly reduce the risk of suicidal behavior. Clinicians should be aware of the warning signs of depression, including changes in mood, appetite, sleep, and social withdrawal.

**Correct Answer:** D. Depression