A 22-year-old male presents to the emergency department with a 12-hour history of periumbilical pain that has now migrated to the right lower quadrant. He has anorexia, nausea, and low-grade fever (38.2°C). On examination, he has tenderness at McBurney's point with rebound tenderness. Ultrasound is performed using graded compression technique. The structure marked **A** in the diagram is identified as a non-compressible blind-ended tubular structure measuring 8 mm in outer diameter. Based on this ultrasound finding, what is the most likely diagnosis?