**Core Concept**
Obsessive-compulsive disorder (OCD) is a chronic mental health condition characterized by the presence of recurring, intrusive thoughts (obsessions) and repetitive behaviors (compulsions) that an individual feels compelled to perform. These obsessions and compulsions are often time-consuming and interfere with daily life.

**Why the Correct Answer is Right**
The correct answer is "D. All" because all the options accurately describe key features of OCD. **Option A:** The irresistible desire to do a thing repeatedly is a hallmark of compulsions in OCD. Individuals with OCD often feel an overwhelming urge to perform specific rituals or behaviors to alleviate their anxiety. **Option B:** Patients with OCD are typically conscious of their disorder and can recognize that their thoughts and behaviors are not rational. However, this awareness does not necessarily help them control their symptoms. **Option C:** Denial is indeed a defense mechanism that some individuals with OCD may use to avoid acknowledging their symptoms or the impact of their disorder on their lives.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not entirely accurate, as some individuals with OCD may experience a sense of resistance to performing their compulsions, although they ultimately feel unable to resist.

**Option B:** While patients with OCD are generally aware of their disorder, some may experience significant distress or feelings of guilt, shame, or embarrassment, which can lead to denial or avoidance of treatment.

**Clinical Pearl / High-Yield Fact**
A key characteristic of OCD is the presence of "just right" feelings, where individuals with OCD feel a sense of relief or satisfaction after performing their compulsions.

**Correct Answer Line**
✓ Correct Answer: D. All