A 32-year-old woman presents to the psychiatric nurse practitioner with a 3-month history of concentration difficulty, irritability, and depression. She has not experienced these symptoms at any other time in her life. During an extensive history inquiry, the woman states that 4 months prior to her visit with the nurse practitioner, she had experienced a short-term flu-like illness and noticed a bull's eye-shaped rash on her calf. She has noted no other symptoms since then, until the mood symptoms began. Her physical examination and laboratory findings are within normal limits. Which of the following is the most likely diagnosis?