An elderly woman and her daughter, who live alone together in social isolation, share an identical unshakeable delusion that their landlord pumps gas into their flat at night. When separated for two weeks during hospital admission, the mother's delusion persists fully while the daughter's belief fades completely. What is the correct interpretation?

• A Both have paranoid schizophrenia requiring lifelong antipsychotics
• B Both have shared psychotic disorder and will relapse identically on reunion
• C The mother has induced delusional disorder; the daughter has late-onset schizophrenia
• D The daughter has induced delusional disorder; the mother has the primary delusion ✓

Explanation

In folie a deux, the dominant partner holds a genuine primary delusional disorder while the dependent partner has an induced delusion that often resolves on separation. The mother retained her delusion when isolated, marking her as the primary case, while the daughter's resolution confirms she was the induced case. Both cannot carry the same diagnosis since the defining feature of induced delusional disorder is dependence on the primary partner's psychopathology.

Reference: Kaplan and Sadock's Synopsis of Psychiatry, 12th ed.

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