History: abdominal pain unspecified (yep - real history). You may want to start out with the big picture on this one.
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Case 26 Answer
Does this help?
yes, it is an abscess but you can be more helpful than this? Perirectal? No. Gluteal? Kind of, but really at the top of the gluteal cleft. Ring any bells?
Answer: Pilonidal abscess
This is a classic location for the pilonidal cyst/sinus spectrum. An ingrown hair typically at the cranial aspect of the intergluteal cleft can cause a foreign body reaction or folliculitis which can worsen into a full fledged abscess. This is a massive example and no doubt is uncomfortable when laying supine, thus the prone exam (and this is exactly how it came across to me, no notes or warning).
The pilonidal sinus is commonly thought of in the neonate/pediatric population however abscess development is actually more common in the patient’s 20′s (male > female)
The main thing you want to differentiate this from is the perianal abscess/fistula. which should approximate the anal sphincter of course.
Brief reference: https://radiopaedia.org/articles/pilonidal-sinus