A 22-year-old university student presents with a 3-week history of diarrhoea, crampy lower abdominal pain and tenesmus. He has 5-6 bloody stools per day and has lost 3 kg. There is mild lower abdominal tenderness. CRP is elevated, and stool cultures including Clostridioides difficile are negative. Flexible sigmoidoscopy demonstrates continuous inflammation from the rectum to the sigmoid colon. What is the most likely diagnosis?