A 40-year-old woman and her younger sister live in isolation after their parents' deaths. Both share an identical, fixed persecutory belief that their former landlord poisons their water supply. When the sisters are admitted separately and kept apart, the younger sister's belief gradually fades, while the elder sister's persists unchanged. What is the most likely diagnosis in the YOUNGER sister?

• A Paranoid schizophrenia
• B Induced delusional disorder (folie a deux) ✓
• C Brief psychotic disorder with marked stressor
• D Substance-induced psychotic disorder

Explanation

Folie a deux, classified as induced delusional disorder in ICD-11, occurs when a delusion held by a dominant partner is adopted by a closely associated, usually submissive and socially isolated person. The diagnostic hallmark demonstrated here is resolution of the induced delusion after separation from the primary case, whose delusion persists because it is endogenous. Paranoid schizophrenia would not remit with separation, brief psychotic disorder lasts under a month but is stress-triggered rather than induced, and no substance is implicated.

Reference: Kaplan and Sadock's Synopsis of Psychiatry, 11th ed.

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