A 45-year-old man is brought to a psychiatric clinic by his family due to his increasingly bizarre behaviour. He insists that he is already dead and that his internal organs have decayed. Despite medical tests proving that his bodily functions are normal, he believes his blood has dried up and his brain no longer exists. His conviction remains unshaken even in the face of contradictory evidence, causing significant distress and impaired daily functioning. Based on these symptoms, what is the most likely diagnosis?

a) Schizoaffective Disorder
b) Delusional Disorder
c) Cotard's Syndrome
d) Schizophrenia