A 17-year-old G1P1001 is now 5 weeks postpartum after a routine vaginal delivery. She calls your office to report a 3-week history of difficulty sleeping and "feeling blue." On further questioning, she reports difficulty concentrating, very poor appetite, occasional wishes that she had never become pregnant, and feelings of guilt about those wishes. She has not left her home in more than a week because she "just can't find the energy to go anywhere." This patient's symptoms are most consistent with: