**Core Concept**
Major depression, also known as unipolar depression, is a complex mood disorder characterized by persistent and intense feelings of sadness, hopelessness, and a loss of interest in activities. It affects various aspects of life, including emotional, cognitive, and physical functioning.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of major depression. Research suggests that major depression is associated with altered levels of neurotransmitters, particularly serotonin, norepinephrine, and dopamine. The monoamine hypothesis of depression proposes that decreased levels of these neurotransmitters contribute to the development of depressive symptoms. Selective serotonin reuptake inhibitors (SSRIs), a class of antidepressants, work by increasing the levels of serotonin in the synaptic cleft, thereby alleviating depressive symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the pathophysiology of major depression. While sleep disturbances are common in patients with depression, they are not a primary cause of the disorder.
**Option B:** This option is incorrect because it is a feature of bipolar disorder, not major depression. Manic episodes are characterized by elevated mood, increased energy, and decreased need for sleep.
**Option C:** This option is incorrect because it is a feature of post-traumatic stress disorder (PTSD), not major depression. Flashbacks and nightmares are common symptoms of PTSD.

**Clinical Pearl / High-Yield Fact**
One key aspect of major depression is that it can affect any individual, regardless of age, sex, or socioeconomic status. Early recognition and treatment are crucial to prevent complications and improve outcomes.

**Correct Answer: C. Manic episodes are a feature of bipolar disorder, not major depression.**