NBME Surgery Shelf Practice Test

Preserving the anal sphincter hinges on tumor height and the ability to achieve clear margins while keeping the sphincter mechanism intact. A low anterior resection with stapled colorectal anastomosis treats mid- to upper-rectal cancers by removing the involved rectal segment and reconnecting the colon to the remaining rectal stump, leaving the anal sphincter muscles intact. This approach maintains continence and avoids a permanent stoma, provided the tumor is not too low or locally advanced and margins can be achieved.

If the tumor cannot be resected with clear margins while sparing the sphincter, more extensive surgery is needed: abdominoperineal resection removes the rectum and anus together, resulting in a permanent colostomy and loss of sphincter function. Transanal excision is reserved for very early, low-lying lesions and isn’t suitable for mid- or upper-rectal cancers due to limited depth of resection and higher risk of incomplete excision. Hemi-colectomy removes part of the colon and does not address the rectal tumor, so it does not preserve the rectal sphincter in this context.