In a patient with a recurrent fistula-in-ano, the external opening lies 4 cm behind the transverse anal line, slightly to the left of the midline. Applying Goodsall's rule, where will the internal opening be found?

• A Radially opposite the external opening in the anal canal
• B In the anterior midline at the level of the anal valves
• C In the posterior midline at the level of the anal valves ✓
• D Directly lateral to the external opening at the same radial position

Explanation

Goodsall's rule states that external openings posterior to the transverse anal line have curved tracks that open into the posterior midline at the dentate line, reflecting flow along the horseshoe-shaped postanal space. Only openings lying directly in the anterior midline open straight radially; other anterior openings follow curved posterior tracks too. This rule guides identification of the internal opening during fistulotomy, and missing it is the commonest cause of recurrence.

Reference: Bailey and Love Short Practice of Surgery, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.