**Core Concept**
Obsessive-Compulsive Disorder (OCD) is a neuropsychiatric condition characterized by the presence of intrusive thoughts (obsessions) and repetitive behaviors (compulsions) that an individual feels compelled to perform. The symptoms of OCD can significantly impair an individual's daily functioning and quality of life.

**Why the Correct Answer is Right**
The most common symptom associated with adult OCD is excessive and persistent cleaning or washing, often referred to as "handwashing." This behavior is frequently driven by the fear of contamination or the presence of germs. The pathophysiology of OCD is thought to involve abnormalities in the functioning of the orbitofrontal cortex and the basal ganglia, which are critical regions involved in executive function, decision-making, and motor control. The serotonin system, particularly the 5-HT1A receptor, has also been implicated in the pathophysiology of OCD.

**Why Each Wrong Option is Incorrect**
**Option A:** While anxiety is a common comorbidity in individuals with OCD, it is not the most common symptom of the disorder. Anxiety can be a symptom of many different conditions, and its presence does not specifically point to OCD.

**Option B:** Hoarding is a symptom of OCD, but it is not the most common symptom. Hoarding can be a distinct subtype of OCD, characterized by the inability to discard or part with possessions.

**Option C:** Checking is another common symptom of OCD, but it is not as prevalent as excessive cleaning or washing. Checking can involve a wide range of behaviors, such as repeatedly checking locks or appliances.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the symptoms of OCD can vary widely between individuals and may change over time. A thorough diagnostic evaluation, including a comprehensive clinical interview and a review of the individual's symptoms and behavior, is necessary to accurately diagnose OCD.

**Correct Answer: D. Excessive and persistent cleaning or washing.**