A severely depressed patient states, with complete and unshakeable conviction, that his bowels have rotted away and no longer exist, holding this as literal fact despite obvious evidence to the contrary, and shows no insight that this could be a subjective feeling rather than a genuine physical absence. What is this?

This is a nihilistic delusion: a fixed belief denying the existence of one's own body, mind, or self, here specifically his bowels, held as literal fact with total conviction, most classically occurring in the context of severe, agitated depression. It is not anosognosia, which denies a specific physical deficit, such as paralysis, rather than existence itself; here the patient denies the very existence of an organ, not a deficit affecting it. It is not depersonalisation, in which the person feels detached or unreal but retains insight that the feeling is subjective, unlike the literal, insight-free conviction described here. And it is not somatoparaphrenia, which involves denying ownership of a body part while still acknowledging it exists, rather than denying the part's existence altogether.