## **Core Concept**
The question describes a scenario involving a 15-year-old boy experiencing repetitive thoughts about dirt and contamination, leading to significant distress, anxiety, and social withdrawal. This presentation suggests an obsessive-compulsive disorder (OCD), characterized by unwanted and intrusive thoughts (**obsessions**) and repetitive behaviors or mental acts (**compulsions**) aimed at reducing anxiety.

## **Why the Correct Answer is Right**
The boy's persistent thoughts about dirt and contamination, despite knowing they are irrational, and his compulsion to think about cleaning, fit the diagnostic criteria for OCD. OCD is marked by the presence of obsessions and/or compulsions that are time-consuming (more than one hour per day) or cause significant distress or impairment in social, occupational, or other areas of functioning. The boy's symptoms have affected his studies and led to social withdrawal, indicating significant impairment.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Without the specific text of Option A, we cannot directly address its inaccuracies. However, common differential diagnoses might include generalized anxiety disorder, which involves excessive worry about everyday things, or specific phobias, which are intense, irrational fears of specific objects or situations. These conditions do not typically involve the same level of compulsive behavior or the specific characteristics of intrusive thoughts and compulsions seen in OCD.
- **Option B:** Similarly, without the text, we can't directly refute it. However, conditions like major depressive disorder could be considered if there were prominent depressive symptoms, which are not described here.
- **Option C:** This option could potentially represent another psychiatric condition. For example, if it suggested a diagnosis like post-traumatic stress disorder (PTSD), it would be incorrect because PTSD involves symptoms that occur after a traumatic event, which is not mentioned in the scenario.

## **Clinical Pearl / High-Yield Fact**
A key feature of OCD is the presence of **insight**, where individuals recognize that their obsessions or compulsions are not real or are unreasonable. This is in contrast to other psychotic disorders where insight is absent. A helpful mnemonic for OCD diagnosis is "Obsessions are unwanted thoughts and Compulsions are repetitive acts," and the impact on daily life is significant.

## **Correct Answer:** D. Obsessive-Compulsive Disorder.