An 82 year old woman, whose husband passed away 6 weeks ago after a long illness, is admitted to the hospital for worsening of her cardiac problems. During the hospital stay, a psychiatrist is invited to consult at the request of her daughters. The daughters think their mother is depressed, because she is tearful, often talks about her dead husband, and states that she had heard his voice several times. She blames herself for not having gone with him on his last visit to his sister, and she stas crying. She has never seen a psychiatrist before and has been "strong" all her life. Which of the following is the most likely diagnosis?