**Core Concept**
Depression is a complex mood disorder characterized by persistent feelings of sadness, hopelessness, and loss of interest in activities. It involves disturbances in neurotransmitter systems, particularly serotonin and norepinephrine, which regulate mood and emotional responses.

**Why the Correct Answer is Right**
Depression is primarily characterized by a combination of biological, psychological, and social factors. The biological factors include an imbalance of neurotransmitters such as serotonin (5-HT) and norepinephrine, which are involved in mood regulation. The reduced activity of 5-HT and norepinephrine is thought to contribute to the development of depressive symptoms. Additionally, genetic and environmental factors also play a significant role in the development of depression.

**Why Each Wrong Option is Incorrect**
**Option A:** While sleep disturbances are common in depression, they are not a defining characteristic of the disorder. Sleep disturbances can occur in various other conditions as well.
**Option B:** Loss of appetite is a common symptom of depression, but it is not the only characteristic of the disorder. Other symptoms such as persistent sadness, hopelessness, and loss of interest in activities are more defining features.
**Option C:** Social withdrawal is a common feature of depression, but it is not a defining characteristic. Social withdrawal can occur in various other conditions as well, such as anxiety disorders.

**Clinical Pearl / High-Yield Fact**
Depression is often comorbid with other medical conditions, such as diabetes, hypertension, and cardiovascular disease. It is essential to screen for depression in patients with these conditions, as untreated depression can worsen the prognosis and outcome.

**Correct Answer:** A.