**Core Concept**
The underlying principle being tested is a mental health disorder characterized by excessive preoccupation with perceived or real threats of contamination, leading to compulsions to perform rituals to alleviate anxiety.

**Why the Correct Answer is Right**
The boy's symptoms, including repetitive thoughts of contamination, anxiety, and compulsions to clean, are classic manifestations of Obsessive-Compulsive Disorder (OCD). This condition is thought to result from an imbalance in neurotransmitters, particularly serotonin, which regulates mood and anxiety. The brain's orbitofrontal cortex and basal ganglia are also implicated in the pathophysiology of OCD. The boy's social withdrawal and impact on studies are common consequences of untreated OCD.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the boy's symptoms do not fit the criteria for a depressive disorder, which typically involves persistent feelings of sadness, loss of interest in activities, and changes in appetite or sleep.

**Option B:** This option is incorrect because the boy's symptoms do not meet the criteria for an Anxiety Disorder Not Otherwise Specified (NOS), which involves excessive worry about everyday things but without the presence of compulsions or rituals.

**Option C:** This option is incorrect because the boy's symptoms do not fit the criteria for a Trauma- and Stressor-Related Disorder, which typically involves symptoms that occur after a traumatic event, such as post-traumatic stress disorder (PTSD).

**Clinical Pearl / High-Yield Fact**
OCD often co-occurs with other mental health disorders, such as depression, anxiety disorders, and attention deficit hyperactivity disorder (ADHD), and should be considered in the differential diagnosis of patients presenting with symptoms of anxiety, compulsions, or rituals.

**Correct Answer: C.**