A 58-year-old man presents with acute onset colicky abdominal pain, nausea, and vomiting. On examination, the structure marked **A** in the diagram is noted, along with visible peristaltic waves and high-pitched tinkling bowel sounds. His abdomen is tender but soft, with no signs of peritonitis. Erect abdominal X-ray shows dilated small bowel loops with air-fluid levels. He had undergone appendectomy 15 years ago. What is the most appropriate initial management?