Classic Derm Findings

BUZZWORD Erythematous rash across both cheeks, sparing the nasolabial folds

• Spares nasolabial folds — this is the key differentiator from rosacea (which doesn't spare them)
• Photosensitive — worsens with sun exposure
• Part of the SLE diagnostic criteria (need 4 of 11)
• Other SLE skin: discoid lupus (scarring), oral ulcers (painless)

BUZZWORD Three concentric color zones: dark center, pale ring, red outer ring

• #1 cause: HSV (herpes simplex) — NOT drugs
• Acral distribution (hands, feet, palms, soles)
• EM minor = just skin. EM major = skin + mucous membranes
• Self-limited — treat the HSV, not the EM

BUZZWORD Single large oval scaly patch followed days later by smaller lesions in a "Christmas tree" pattern on the back

• Herald patch comes first (the "herald" announces the army)
• Smaller lesions follow the skin cleavage lines → Christmas tree on back
• Associated with HHV-6 and HHV-7
• Self-limited over 6-8 weeks, no treatment needed
• Young adults, common in spring/fall

BUZZWORD Clear vesicles on an erythematous base, lesions in different stages simultaneously

• Different stages at once = pathognomonic (macules, papules, vesicles, crusts all present)
• Starts on trunk, spreads centrifugally
• Vs smallpox: all lesions in the SAME stage, starts on face/extremities (centripetal → centrifugal)
• Complication in adults: varicella pneumonia
• Complication in kids on aspirin: Reye syndrome

BUZZWORD Gentle lateral pressure on normal-appearing skin causes the epidermis to separate and slough off

• Pemphigus vulgaris: Nikolsky (+), flaccid blisters, intraepidermal (superficial split)
• Antibodies against desmoglein 3 (desmosome component)
• Oral mucosa involved FIRST in most patients
• Vs bullous pemphigoid: Nikolsky (−), tense blisters, subepidermal (deeper split)
• Pemphigoid: anti-hemidesmoglein (BP180/BP230), elderly patients, pruritic

BUZZWORD Well-demarcated erythematous plaques with silvery-white scales; scraping reveals pinpoint bleeding (Auspitz sign)

• Extensor surfaces: elbows, knees, scalp, sacrum
• Nail pitting is classic (also: oil spots, onycholysis)
• Koebner phenomenon: lesions appear at sites of trauma/injury
• Histology: Munro microabscesses, parakeratosis, epidermal hyperplasia
• Associated with psoriatic arthritis (DIP joints, dactylitis, pencil-in-cup deformity)

BUZZWORD Purple papules over MCP/PIP joints (Gottron) + purple-lilac eyelid discoloration (heliotrope)

• Proximal muscle weakness + elevated CK
• Anti-Jo-1 antibodies (antisynthetase syndrome)
• Anti-Mi-2 (classic DM), anti-MDA5 (amyopathic DM)
• Shawl sign: V-shaped photodistributed rash on upper back/chest
• Screen for malignancy — ovarian, lung, GI cancers in adults with new-onset DM

BUZZWORD ≥6 cafe-au-lait spots >5mm (prepubertal) or >15mm (postpubertal)

• NF1 diagnostic criteria (need 2+): cafe-au-lait spots, neurofibromas (≥2), axillary/inguinal freckling, optic glioma, Lisch nodules (iris hamartomas), osseous lesion, first-degree relative
• Chromosome 17, neurofibromin gene (tumor suppressor)
• Autosomal dominant
• Risk of malignant peripheral nerve sheath tumors

BUZZWORD Hypopigmented macules best seen under Wood lamp (UV light)

• Classic triad: seizures, intellectual disability, angiofibromas (adenoma sebaceum)
• Other skin: shagreen patches (leathery plaques on lower back), periungual fibromas
• Cardiac rhabdomyomas (in neonates — most common cardiac tumor in infants)
• Renal angiomyolipomas, cortical tubers (brain)
• TSC1 (hamartin) or TSC2 (tuberin) — tumor suppressors

BUZZWORD New lesions appearing at sites of skin trauma (scratches, surgical scars, tattoos)

• The Big 3 that Koebner: Psoriasis, Lichen planus, Vitiligo
• Also seen in: molluscum contagiosum, warts, lichen nitidus
• Isomorphic response = same as Koebner (different name, same concept)
• Boards tests this as: "patient develops new psoriatic plaques along a recent surgical scar"

BUZZWORD Rubbing a skin lesion causes urtication (wheal and flare) due to mast cell degranulation

• Urticaria pigmentosa = most common form in kids (brown-red macules/papules)
• Systemic mastocytosis: flushing, diarrhea, hypotension, hepatosplenomegaly
• Elevated serum tryptase
• Histology: increased mast cells (toluidine blue or Giemsa stain, metachromatic granules)

BUZZWORD Intensely pruritic grouped vesicles on extensor surfaces (elbows, knees, buttocks)

• "Herpetiformis" = looks like herpes (grouped vesicles) but is NOT herpes
• IgA deposits at dermal papillae on immunofluorescence
• Anti-tissue transglutaminase (anti-tTG) antibodies = same as celiac
• Treatment: dapsone (rapid relief) + gluten-free diet
• Most patients have celiac disease, even if asymptomatic GI

BUZZWORD Tender, red nodules on the anterior shins (panniculitis of the subcutaneous fat)

• Most common causes: sarcoidosis, IBD (especially Crohn's), strep pharyngitis, OCPs, sulfonamides
• Septal panniculitis — inflammation of fat septae, NO vasculitis
• Self-limited — treat the underlying cause
• Biopsy: Miescher radial granulomas in the septae

BUZZWORD Non-blanching, raised, purplish lesions on lower extremities

• "Palpable" = raised = vasculitis until proven otherwise
• In a child: IgA vasculitis (HSP) — palpable purpura + abdominal pain + arthralgia + hematuria
• IgA deposits in skin and kidney (IgA nephropathy component)
• In adults: drug reaction, infection, or autoimmune vasculitis
• Non-palpable purpura = thrombocytopenia, not vasculitis

SIGN OVERLAP These two get confused because both involve mucous membranes

• SJS: Drug-induced (sulfa, phenytoin, allopurinol, carbamazepine, lamotrigine). Epidermal detachment <10% BSA. Atypical target lesions. Nikolsky (+).
• TEN: Same as SJS but >30% BSA detachment. Medical emergency. Mortality 25-35%.
• Behcet's: Recurrent oral + genital ulcers + uveitis. HLA-B51. Silk Road countries. Pathergy test (+).
• SJS/TEN is acute and drug-related. Behcet's is chronic and autoimmune.

BUZZWORD V-shaped photodistributed rash + hyperkeratotic, cracked skin on fingertips

• Shawl sign = rash draped across upper back/shoulders like a shawl
• Mechanic's hands = rough, cracked, hyperkeratotic lateral fingers (antisynthetase syndrome)
• Anti-Jo-1 is the antibody for antisynthetase syndrome
• Antisynthetase syndrome: mechanic's hands + ILD + myositis + arthritis + Raynaud's + fever

BUZZWORD Golden/honey-colored crusts on an erythematous base, usually perioral in children

• Non-bullous impetigo (70%): S. aureus or GAS → honey crusts
• Bullous impetigo: S. aureus exfoliative toxin → flaccid bullae
• Complication of GAS impetigo: post-strep glomerulonephritis (NOT rheumatic fever — skin strep doesn't cause RF)
• Treatment: topical mupirocin (mild), oral cephalexin (extensive)

BUZZWORD Sandpaper-textured diffuse erythematous rash, red tongue with prominent papillae, pastia lines

• Caused by GAS erythrogenic toxin (superantigen)
• Follows GAS pharyngitis (NOT skin infection)
• Pastia lines: linear petechiae in skin folds (axillae, antecubital)
• Circumoral pallor (pale around the mouth)
• Desquamation during recovery (starts at fingertips)

BUZZWORD Bright red facial erythema ("slapped cheeks") followed by lacy/reticular rash on trunk and extremities

• Parvovirus B19 (ssDNA virus, smallest DNA virus)
• Infects erythroid precursors → temporary halt in RBC production
• Dangerous in sickle cell: aplastic crisis (reticulocyte count drops to zero)
• Dangerous in pregnancy: hydrops fetalis
• Dangerous in immunocompromised: pure red cell aplasia

HIGH-YIELD COMPARISON

• BCC: Most common skin cancer. Pearly, translucent papule with telangiectasias and rolled borders. Sun-exposed areas. Almost NEVER metastasizes. Upper lip.
• SCC: Second most common. Scaly, ulcerated nodule. Can metastasize. Arises from actinic keratoses. Lower lip. Associated with immunosuppression.
• Melanoma: Most deadly. ABCDE criteria (Asymmetry, Border irregularity, Color variation, Diameter >6mm, Evolving). Breslow depth = most important prognostic factor.