**Core Concept**
Delusions are fixed, false beliefs that are not amenable to change in light of conflicting evidence. They are a hallmark symptom of various psychiatric disorders and are often used as a diagnostic criterion. Delusions can be categorized into different types, including persecutory, grandiose, and delusions of reference.

**Why the Correct Answer is Right**
Obsessive-Compulsive Disorder (OCD) is characterized by intrusive thoughts (obsessions) and repetitive behaviors (compulsions) aimed at reducing anxiety associated with these thoughts. While individuals with OCD may experience anxiety and distress related to their obsessions, they typically do not develop fixed, false beliefs that are characteristic of delusions. In contrast, delusions are a key feature of psychotic disorders, such as schizophrenia, where individuals may experience a break from reality. Delirium and alcohol withdrawal can also involve delusions, particularly in the context of altered mental status and cognitive impairment.

**Why Each Wrong Option is Incorrect**
**Option A:** Delirium is a state of altered consciousness characterized by confusion, disorientation, and fluctuating levels of consciousness. Delusions can occur in delirium, especially in the context of cognitive impairment and altered mental status.

**Option B:** Schizophrenia is a chronic and severe mental disorder that affects an individual's thought, behavior, and emotional processes. Delusions are a core feature of schizophrenia, and individuals may experience various types of delusions, including persecutory and grandiose delusions.

**Option D:** Alcohol withdrawal can involve delirium tremens, a severe and potentially life-threatening condition characterized by altered mental status, confusion, and hallucinations. Delusions can occur in the context of alcohol withdrawal, particularly in individuals with a history of chronic alcohol use.

**Clinical Pearl / High-Yield Fact**
It is essential to distinguish between delusions and obsessions, as they represent distinct psychological phenomena. While both can be distressing, delusions are fixed, false beliefs that are not amenable to change, whereas obsessions are intrusive thoughts that an individual may attempt to suppress or neutralize through compulsions.

**✓ Correct Answer: C. OCD**