**Core Concept**
Folie a deux is a psychiatric condition characterized by the development of a shared delusion between two individuals, typically in a close relationship such as a couple or family member. It involves the transmission of delusional beliefs from one person to another, reflecting a form of psychotic disorder with social and psychological contagion.

**Why the Correct Answer is Right**
Folie a deux is clinically defined as a shared delusion disorder where one person develops a delusion that is adopted by another, often due to strong interpersonal bonds. The delusion is not merely influenced by the patient but is actively shared and believed by both individuals. This phenomenon is most commonly seen in patients with schizophrenia or psychotic disorders, especially in close relationships, and is distinct from other conditions like OCD or hysteria. The term "shared delusion" accurately captures the essence of the condition.

**Why Each Wrong Option is Incorrect**
Option A: OCD (Obsessive-Compulsive Disorder) involves recurrent, intrusive thoughts and compulsive behaviors, not shared delusions. It is a non-psychotic disorder and does not involve delusional transmission.
Option C: Hysteria (now termed conversion disorder) is a functional neurological condition with somatic symptoms without organic cause, unrelated to delusional sharing.
Option D: Neurasthenia is an outdated diagnosis referring to fatigue and stress-related symptoms, with no link to delusional sharing or psychosis.

**Clinical Pearl / High-Yield Fact**
Folie a deux is rare but clinically significant—always consider it in cases of delusional belief shared between partners, especially in psychiatric patients with schizophrenia spectrum disorders. Early recognition can prevent worsening of symptoms and improve treatment outcomes.

✓ Correct Answer: B. Shared delusion disorder