**Question:** A 20-year old man has presented with increased alcohol consumption and sexual indulgence, irritability, lack of sleep, and not feeling fatigued even on prolonged periods of activity. All these changes have been present for 3 weeks. The most likely diagnosis is:

A. Depression
B. Anxiety
C. Substance Use Disorder
D. Hypothyroidism

**Correct Answer:** C. Substance Use Disorder

**Core Concept:**
Substance Use Disorder (SUD) is a chronic condition characterized by problematic patterns of substance use leading to clinically significant impairment or distress. It includes two main categories: alcohol use disorder (AUD) and drug use disorder (DUD). AUD is a group of conditions characterized by a pattern of alcohol use leading to clinically significant impairment or distress.

**Why the Correct Answer is Right:**
The clinical presentation of the patient described in the question aligns with the criteria for alcohol use disorder (AUD). The patient exhibits several symptoms that are consistent with AUD:

1. Increased alcohol consumption: The patient is consuming more alcohol than usual to achieve a desired effect.
2. Sexual indulgence: The patient shows excessive sexual activity due to alcohol's effects on dopamine release and central nervous system.
3. Irritability: Alcohol can cause irritability as a result of its effects on central nervous system and neurotransmitters.
4. Lack of sleep: Alcohol can disrupt sleep patterns, leading to insomnia or excessive sleepiness.
5. Not feeling fatigued after prolonged activity: Alcohol has a depressant effect on the central nervous system, causing decreased sensitivity to fatigue.

**Why Each Wrong Option is Incorrect:**
A. Depression (Option A): The main symptoms of depression include feelings of sadness, hopelessness, and suicidal ideation, none of which are present in the patient described.
B. Anxiety (Option B): The patient does not exhibit symptoms consistent with anxiety disorders such as panic attacks or generalized anxiety.
D. Hypothyroidism (Option D): The symptoms described are more consistent with substance use disorders, not thyroid dysfunction.

**Clinical Pearl:**
Recognizing and diagnosing Substance Use Disorders (SUD) is crucial in clinical practice, especially in younger individuals presenting with behavioral changes and psychiatric symptoms. A thorough history and physical examination, along with appropriate investigations, should be performed to confirm the diagnosis and guide management strategies.