I think there are a few inherent differences between GI/urology vs dermatology:
1) Sheer number of diagnoses that are commonly encountered. Dermpath has hundreds of commonly-encountered diagnoses, and countless less-commonly-encountered variations thereof. Also, subtle histopath differences in inflammatory disorders often matter a lot clinically (eg. psoriasis vs psoriasiform dermatitis, which have completely different clinical implications and treatment options). There are simply fewer such problems in GI or urologic path.
2) Dermatologists know more about their organ's histopath than any other medical or surgical specialty, so they can more easily detect bad pathologists.
3) Dermatologists are some of the most type-A people and are into minutiae. So, they are more likely to take offense to any type of pathology misread, including ones that may not matter clinically (eg., erythema annulare centrifugum vs. pityriasis rosea-like eruption).
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