Small Bowel Obstruction Transition Point Adhesion

A 54-year-old woman presents with 18 hours of colicky central abdominal pain, bilious vomiting, and obstipation. She has a history of open appendicectomy 12 years ago and total abdominal hysterectomy 4 years ago. Examination reveals dehydration, tachycardia (HR 108), and a distended tender abdomen with high-pitched tinkling bowel sounds. No peritonitis or hernia is noted. Contrast-enhanced CT abdomen and pelvis shows dilated proximal small bowel loops (>3 cm) with collapsed distal bowel. The structure marked **B** in the diagram—the transition point with abrupt caliber change—is identified in the pelvis without evidence of mass, hernia, or intussusception. What is the most likely diagnosis and primary mechanism of obstruction in this patient?