A 26-year-old woman comes to the emergency room after a suicide attempt. She delivered her first child four days ago. She says that soon after the delivery God began appearing before her and speaking to her directly, telling her that she could never be a good mother, so she'd best spare her child and partner by killing herself. Her partner attests that she has “not been herself” since she got home from the hospital; that for the last three days she has been behaving quite erratically, talking to herself, occasionally saying completely unintelligible things, not sleeping, bathing, or grooming herself; and fluctuating between panic, rage, and despair. She refuses to be anywhere near the baby. There is no past or family history of psychiatric illness. A general and systemic examination reveals no abnormalities. There is no history of substance misuse. A mental status examination shows that she is well-oriented to time, place, and person. What is the most likely diagnosis?