A 40-year-old woman and her 17-year-old daughter, who live in isolation after migrating, share an identical persecutory delusion that their landlord is poisoning the water supply. When separated for two weeks, the daughter's delusion resolves completely while the mother remains deluded. The most likely diagnosis is:
- A Shared psychotic disorder (folie a deux) ✓
- B Paranoid schizophrenia in both individuals
- C Delusional disorder, persecutory type, in both
- D Brief psychotic disorder with marked stressor
Correct answer: A. Shared psychotic disorder (folie a deux)
Explanation
Folie a deux is a shared delusional disorder in which a dominant primary case transmits a delusion to a submissive partner, usually within a close relationship and social isolation. Resolution of the delusion on separation confirms the induced nature in the secondary case, who is typically younger or more dependent. The persistence of the delusion in the mother identifies her as the primary case with delusional disorder, while the daughter does not have independent schizophrenia.
Reference: Kaplan and Sadock's Synopsis of Psychiatry, 12th ed.
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