A 53 year old female lawyer who has been married for 20 years comes to her physician's office because she has not felt "up to par" over the past 2 months. She is married, has two grown sons, and has a good practice. In the past couple of weeks, she has stopped taking care of her appearance and has frequently called in sick to work, when she actually has been having difficulty getting out of bed. She states that she has lost her appetite recently and her interest in sex with her husband has decreased considerably. She recently told her husband that at times she wonders whether she should go on living. She denies any history of drug or alcohol abuse, and a complete physical examination 3 months earlier showed her to be in good health. Which of the following is the mostly likely diagnosis?