A 28-year-old man comes to the office saying that he is "just not feeling like myself." This feeling began soon after his older brother passed away from cancer 3 months ago. The patient is no longer interested in work or going out with friends on weekends. He falls asleep relatively easily at night but awakens very early in the morning and is unable to fall back asleep. He feels "down in the dumps," especially when he first wakes up, but says that his mood improves a little during the day. The patient often thinks about the time he spent with his brother growing up and how he could always depend on him. He feels guilty about his brother's death and believes it to be his fault "because I am worthless and could not love him enough." The patient's poor energy and concentration have started to affect his performance at work, and he frequently thinks that he should just quit "because I am bringing everyone down." He has been eating less and has lost 5.4 kg (12 lb) over the past few weeks. He has had no suicidal thoughts. The patient has no psychiatric history other than counseling as a teenager when his parents divorced. Which of the following is the most likely diagnosis?