Question

A 32-year-old man comes to the outpatient clinic reporting a 4-week history of progressive lower back pain. He states the pain is most severe upon awakening, lasts for about 90 minutes, and gradually improves after he takes a warm shower. Over the last week, he has also developed pain and swelling in his right knee and left heel. His past medical history is notable for a severe episode of watery diarrhea with abdominal cramping six weeks ago that resolved spontaneously after three days. He has no prior surgical history. He takes ibuprofen as needed, which provides partial relief, and uses no daily prescription medications or herbal supplements. Social history is significant for working as an accountant with prolonged daily sitting, occasional alcohol consumption, and no history of smoking or illicit drug use. His temperature is 37.2°C (99.0°F), blood pressure is 122/78 mmHg, and pulse is 78/min. Physical examination reveals tenderness over the bilateral sacroiliac joints, a moderate effusion of the right knee, and marked tenderness at the insertion of the left Achilles tendon on the calcaneus. Which of the following is the most likely diagnosis?