A patient briefly mentions having "seen something odd" in low light recently. Which single follow-up question would most usefully help establish whether this was an illusion?
Directly asking whether a real object was present and whether the perception corrected itself on closer inspection targets exactly the two features that define an illusion. Fear, prior occurrences, and emotional context are reasonable background but do not themselves establish whether a real stimulus was present, which is the categorical feature that separates an illusion from a hallucination.
Key discriminating feature: The defining test for an illusion is whether a real stimulus was present and whether the misperception resolved on closer attention — asking about both directly is the most efficient way to confirm the pattern.
References
- Casey, P. & Kelly, B. (2019). Fish's Clinical Psychopathology, 4th ed. Ch. 3, 'Disorders of Perception'.