A 35-year-old woman presents with severe anal pain during defecation and visible blood on toilet paper for 3 weeks. On examination, a visible tear is seen in the anal canal at the posterior midline. In contrast, a 42-year-old man with a 2-year history of recurrent anal discharge and a palpable nodule in the perianal skin is found to have a tract with an external opening. Which single feature best distinguishes an anal fissure from an anal fistula?