A severely depressed patient states, with complete conviction, that his legs no longer exist at all, insisting this is a literal fact despite clear visual evidence to the contrary, and that his body has effectively ceased to be. Before revealing the answer: is this body image disturbance?

No, this is more consistent with a nihilistic delusion than with body image disturbance. Body image disturbance is typically a distorted sensory experience of size, shape, position, or ownership; here, the fixed, delusional conviction that the legs and body do not exist at all, held with complete certainty despite clear contrary evidence, is the defining pattern of a nihilistic delusion rather than a distortion of body image.