**Core Concept**
Obsessive-Compulsive Disorder (OCD) is characterized by the presence of intrusive, recurring thoughts (obsessions) and repetitive behaviors (compulsions) aimed at reducing anxiety associated with these thoughts. In OCD, individuals often engage in excessive checking behaviors to alleviate feelings of uncertainty or danger.

**Why the Correct Answer is Right**
The gentleman's repeated thoughts about not locking his car door properly and frequent checks while driving are classic examples of an obsession. His repetitive checking behavior in his house is also a manifestation of a compulsion. This pattern of intrusive thoughts and compulsive behaviors is a hallmark of OCD, which is a common psychiatric disorder. The underlying pathophysiology of OCD involves abnormalities in the orbitofrontal cortex, anterior cingulate cortex, and basal ganglia, which are critical in the regulation of executive functions, emotion, and motor control.

**Why Each Wrong Option is Incorrect**
**Option A:** Somatoform disorder is a psychiatric condition characterized by physical symptoms that cannot be explained by a medical condition. The gentleman's symptoms, however, are clearly related to his thoughts and behaviors, making somatoform disorder an incorrect diagnosis.
**Option B:** OPCD is not a recognized psychiatric diagnosis. It appears to be a distractor and is not relevant to the gentleman's symptoms.
**Option C:** Phobia is an anxiety disorder characterized by an excessive or irrational fear of a specific object, situation, or activity. The gentleman's symptoms are not related to a specific object or situation that elicits fear, but rather to a recurring thought and compulsive behavior.

**Clinical Pearl / High-Yield Fact**
It's essential to distinguish between OCD and other anxiety disorders, such as generalized anxiety disorder and phobias, which may present with similar symptoms. A thorough psychiatric evaluation, including a detailed history and mental status examination, is necessary to establish a diagnosis of OCD.

**✓ Correct Answer: A. OCD**