A 56- year- old biologist has had a very successful career, publishing extensive research on the aging process. Now he believes he has found the key to eternal life. For the past several months, he has been quite secretive and refuses to come to work group meetings regarding upcoming projects. He has installed a dead bolt on his office at home and insists that extra security personnel be hired to guard his research lab. He refuses to present his material or discuss it with his colleagues or family, claiming the knowledge is too powerful to be made public. He is not having any visual or auditory hallucinations, and he is able to support his statements with logical arguments. He has a hx of depression. What is the likely diagnosis?

A. Cocaine intoxication, without perceptual distrubances.
B. Delusional disorder, grandiose type.
C. Major depressive disorder with psychotic features.
D. Schizophrenia.