General: areas of friction/pressure that undergo epidermal thickening, very common 


HPI: patients have discomfort with walking or standing, maybe uncomfortable shoes, lots of barefoot walking


Physical exam: thick, brown/yellow or white/gray plaque or nodule, circular, well-circumscribed, horn-like, if one were to scrape the surface it would reveal a clear center with intact skin lines 


Differential: warts (may not have a central clearing with intact skin lines)


Histology: hyperkeratosis, thickened epidermal layer, however biopsy is unnecessary 


Complications: can persist, can get bursitis, infection, and even gangrene in at-risk patients (neurologic dysfunction from diabetes, arteriosclerosis)


Treatment: (1) paring with scalpel, (2) 40% salicylic acid  plaster to soften (keratolytic), (3) correct underlying cause of excessive pressure or friction, can also do surgery if anatomical skeletal deformity exists 


References: AAD Basic Dermatology Curriculum, Dermatology by Bolognia et. al., Lookingbill and Mark's Principles of Dermatology, First Aid USMLE Step 1 2020, First Aid USMLE Step 2 CK