Perianal Fistula-in-Ano

A 38-year-old man presents with an 8-month history of recurrent perianal pain and purulent discharge from an external opening in the left posterior quadrant. Digital rectal examination reveals an indurated cord extending from the external opening toward the anal canal with tenderness at the dentate line. Contrast-enhanced CT of the pelvis with thin coronal reformats is obtained. The structure marked **A** in the diagram shows a linear enhancing tract arising from the dentate line, crossing the external anal sphincter at the mid-sphincteric level, and terminating in the left ischiorectal fossa. Based on Parks classification and the level of sphincter involvement demonstrated by structure **A**, which surgical approach is most appropriate for this patient?