A 28-year-old woman from Mumbai presents with a 6-month history of bloody diarrhea, abdominal cramping, and weight loss of 4 kg. She reports 6–8 loose stools daily with visible blood and mucus. On examination, she is afebrile, with mild left lower quadrant tenderness. Laboratory investigations show hemoglobin 9.2 g/dL, ESR 62 mm/h, CRP 8.5 mg/dL, and fecal calprotectin 450 µg/g. Colonoscopy reveals continuous mucosal inflammation, friability, and ulceration limited to the rectum and sigmoid colon, with normal mucosa proximally. Multiple biopsies show crypt distortion, increased chronic inflammation in the lamina propria, and absence of granulomas. What is the most likely diagnosis?