**Core Concept**
Othello syndrome is a rare psychiatric condition characterized by delusional jealousy, where a person believes their partner is unfaithful, despite no evidence. It is a subtype of delusional disorder, specifically delusional jealousy, and is named after Othello from Shakespeare, who believed his wife was unfaithful.

**Why the Correct Answer is Right**
Othello syndrome involves persistent, fixed delusions of infidelity or betrayal by a spouse or partner, often leading to intense jealousy and suspicious behavior. These delusions are not based on reality and are not due to substance use or medical illness. The pathophysiology involves dysfunction in the brain’s limbic and prefrontal regions, which regulate emotional regulation and reality testing. It is distinct from other delusional disorders and is clinically recognized in DSM-5 as delusional disorder with jealous content.

**Why Each Wrong Option is Incorrect**
Option A: Capgras syndrome involves the delusion that a familiar person has been replaced by an imposter, not jealousy or infidelity.
Option C: Hypochondrial paranoid disorder (now called delusional disorder with hypochondriacal content) involves persistent belief in having a serious illness, not jealousy.
Option D: Declerambault's syndrome is a delusion of being loved or admired by someone else, not jealousy or infidelity.

**Clinical Pearl / High-Yield Fact**
Othello syndrome is a classic example of delusional jealousy and is often seen in older adults or individuals with schizophrenia or other psychotic disorders. It is a red flag for underlying psychiatric illness and requires thorough psychiatric evaluation and possible treatment with antipsychotics.

✓ Correct Answer: B. Othello syndrome