A patient describes a fixed delusional belief. Which single question would most usefully help establish whether this is a secondary delusion rather than a primary one?
Asking specifically about the person's mood or other experiences at the time the delusion first began to form is what tests whether the belief can be traced back to, and understood as arising from, another underlying morbid experience — exactly the defining feature of a secondary delusion. Strength of conviction, family history, and hypothetical reactions to disproof don't test this specific origin question.
Key discriminating feature: The defining feature of a secondary delusion is that it can be traced back to, and understood in light of, a preceding underlying experience (mood, hallucination, personality), so asking directly about what preceded the belief's formation is the most useful test.
References
- Casey, P. & Kelly, B. (2019). Fish's Clinical Psychopathology, 4th ed. Ch. 4, 'Disorders of Thought and Speech'.