A 66-year-old woman with a dense left-sided paralysis is asked to raise both arms. She lifts only her right arm, insists she has raised both, and when the doctor points out that her left arm never moved, she calmly says it "must just be a bit tired today" and changes the subject, showing no concern at all. What is this?

This is anosognosia: an unawareness of an objectively demonstrable physical deficit, in a person who is otherwise alert and coherent, paired with a striking lack of emotional concern. Two features rule out the alternatives here. It is not denial as a defence mechanism, because denial concerns the emotional significance of a threatening reality (such as a diagnosis) rather than unawareness of a physical sign, and it does not typically come with this degree of indifference. It is not somatoparaphrenia, which is denial that a limb belongs to oneself at all, not denial that the limb is weak. And it is not a nihilistic delusion, which would involve an elaborated false belief system rather than unawareness of one specific, demonstrable deficit.