Musculoskeletal, Skin, and Subcutaneous Tissue
Bone and joint
Fractures are described by location, displacement, angulation, and open versus closed status; healing proceeds through hematoma, soft callus, hard callus, and remodeling. Compartment syndrome: pain out of proportion, pain with passive stretch, and paresthesias - pulselessness is a late finding; treatment is fasciotomy. Osteomyelitis: Staphylococcus aureus is most common overall, Salmonella in sickle cell disease, and broader coverage is needed in diabetics and injection drug users. Septic arthritis: acute monoarthritis with fever; aspiration shows over 50,000 white cells and often a positive Gram stain. Osteoporosis: DEXA T-score of −2.5 or lower; treat with bisphosphonates plus calcium and vitamin D. Osteomalacia: low vitamin D, bone pain, pseudofractures. Paget disease: elevated ALP with normal calcium and a mosaic trabecular pattern.
Arthritis patterns
Osteoarthritis: weight-bearing joints, morning stiffness under 30 minutes, osteophytes, joint space narrowing. Rheumatoid arthritis: symmetric small-joint disease, morning stiffness over 1 hour, anti-CCP, ulnar deviation, extra-articular involvement. Gout: negatively birefringent needle-shaped crystals, podagra; treat acutely with NSAIDs or colchicine. Pseudogout: positively birefringent rhomboid crystals.
Skin
Pemphigus vulgaris: intraepidermal, flaccid bullae, positive Nikolsky sign, anti-desmoglein antibodies. Bullous pemphigoid: subepidermal, tense bullae, negative Nikolsky sign, linear IgG at the basement membrane. Psoriasis: silvery scale on extensor surfaces with Auspitz sign. Eczema: flexural and pruritic. Cellulitis versus erysipelas (sharply demarcated and raised). Necrotizing fasciitis: pain out of proportion, crepitus, surgical emergency. Melanoma: ABCDE criteria; basal cell carcinoma: pearly papule with telangiectasia; squamous cell carcinoma: keratotic lesion on sun-exposed skin.
Rule of thumb
Burns by the rule of nines: each arm 9%, each leg 18%, anterior trunk 18%, posterior trunk 18%, head 9%, perineum 1%. Parkland formula gives 4 mL × kg × %TBSA of lactated Ringer over 24 hours, half in the first 8 hours.