You suspect a patient's longstanding suspicious pattern may have crystallised into a fixed delusional belief rather than remaining at the level of a personality trait. Which single question would most usefully test this?

Directly testing whether a specific belief would, even in principle, respond to disconfirming evidence is the most useful way to establish whether it has crossed from an overvalued suspicious personality trait into a fixed delusion. Duration of the general pattern, family agreement, or past real events are useful background but do not themselves test the fixity of any one specific belief the way this question does.

Key discriminating feature: Testing openness to disconfirming evidence directly probes fixity, which is exactly what separates a personality-level overvalued suspicious pattern (paranoid personality disorder) from a specific, fixed delusional belief that has decompensated out of it — the same fixity test used elsewhere for delusions generally applies here.

Clinical interviewing principle: When a longstanding personality trait is suspected of having crystallised into a specific delusion, test the specific belief's fixity directly, rather than relying only on the general duration or pervasiveness of the underlying suspicious pattern.