A 28-year-old male presents with abdominal pain and bloody purulent stools for 8 days, and joint swelling/pain for 3 days. Physical exam: bilateral conjunctival congestion, swelling of the left knee joint (+ patellar tap test), erythematous skin lesions on soles with scattered yellow-brown vesicles, crusts, and ulcers. Urinalysis shows 4-5 WBCs/HPF. The most likely diagnosis is: