**Core Concept**
Obsessive-Compulsive Disorder (OCD) is a chronic mental health condition characterized by unwanted and intrusive thoughts (obsessions) and repetitive behaviors or mental acts (compulsions) aimed at reducing anxiety associated with these thoughts. The pathophysiology of OCD involves abnormalities in brain regions, including the orbitofrontal cortex, anterior cingulate cortex, and basal ganglia, with altered neurotransmitter systems, particularly serotonin.

**Why the Correct Answer is Right**
The core features of OCD include:
- Recurrent, intrusive thoughts (obsessions), such as fear of contamination or harm to oneself or others.
- Repetitive behaviors or mental acts (compulsions), such as excessive cleaning, checking, or counting, aimed at reducing anxiety.
- Significant distress or impairment in social, occupational, or other areas of functioning.
- The presence of obsessions and compulsions for at least 1 hour per day.
The correct answer will include these essential features.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option may include a symptom or feature that is commonly associated with OCD but is not a core characteristic. For example, "Hoarding" is a symptom often seen in OCD, but it is not a universal feature of the disorder.

**Option B:**
This option may include a feature that is not specific to OCD or is more characteristic of another psychiatric condition. For example, "Hallucinations" are more commonly associated with psychotic disorders, such as schizophrenia.

**Option C:**
This option may include a feature that is not a primary characteristic of OCD or is more relevant to another condition. For example, "Mood swings" are more commonly associated with mood disorders, such as bipolar disorder.

**Option D:**
This option may include a feature that is not a core characteristic of OCD or is more relevant to another condition. For example, "Sleep disturbances" are more commonly associated with anxiety disorders, such as generalized anxiety disorder.

**Clinical Pearl / High-Yield Fact**
The Yale-Brown Obsessive Compulsive Scale (Y-BOCS) is a widely used assessment tool for OCD, evaluating the severity of obsessions and compulsions. A score of 16 or higher indicates severe symptoms.

**Correct Answer: A. Hoarding**