The hallmark clinical sign of acute appendicitis is tenderness at McBurney's point, located one-third the distance from the anterior superior iliac spine to the umbilicus. This area corresponds to the anatomical position of the inflamed appendix. Palpation here typically elicits localized pain due to peritoneal irritation. Identification of McBurney's point tenderness is crucial for surgical decision-making and differentiates appendicitis from other acute abdominal conditions. This sign guides timely appendectomy, preventing complications like perforation.

Reference: Sabiston Textbook of Surgery, Townsend et al., 21st Edition.