A 50-year-old man presents with a 6-month history of itching and a palpable lump at the anal verge that appears during straining and retracts spontaneously. Anoscopy shows internal hemorrhoids above the dentate line. A 38-year-old woman with a 2-year history of recurrent anal pain, purulent drainage, and a visible opening in the perianal skin is diagnosed with an anal fistula on MRI. Which feature most reliably distinguishes internal hemorrhoids from an anal fistula?