A 35-year-old man has recurrent perianal discharge. Examination reveals an external opening 4 cm behind the anus in the posterior midline. According to Goodsall's rule, where will the internal opening be found?

• A In the posterior midline at the anal verge level
• B At the dentate line directly opposite the external opening laterally
• C Radially opposite the external opening in the anterior midline
• D At the dentate line in the posterior midline ✓

Explanation

Goodsall's rule states that if an imaginary transverse line is drawn across the anus, an external opening posterior to this line tracks along a curved path to an internal opening in the posterior midline at the dentate line. Openings anterior to the line open radially into the anal canal directly opposite. The exception is an anterior external opening more than 3 cm from the anal verge, which may still track to the posterior midline.

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

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Written and medically reviewed by the StethoPrep medical team.