Mastering Dermatopathology: A Pattern-Recognition Cheat Sheet for Board Exams

1. Psoriasiform Dermatitis

• Pattern: Epidermal hyperplasia, elongated rete ridges.

• Differential & Tie-Breaker (One Discriminator):

  • Psoriasis: Neutrophils in stratum corneum (Munro microabscesses).

  • Chronic Spongiotic Dermatitis (e.g., Atopic Dermatitis): Spongiosis (epidermal edema) present.

  • Lichen Simplex Chronicus: Hyperkeratosis, vertical papillary dermal fibrosis.

  • Pityriasis Rubra Pilaris: Alternating ortho- and parakeratosis horizontally, follicular plugging.

  • Mycosis Fungoides (patch/plaque): Atypical lymphocytes lined up in basal layer (epidermotropism).

  • Secondary Syphilis: Plasma cell-rich dermal infiltrate.

  • Dermatophytosis: Septate hyphae in stratum corneum (PAS+).

  • Seborrheic Dermatitis: Spongiosis focused around follicular infundibula.

  • Reactive Arthritis (Keratoderma Blennorrhagicum): Spongiform pustules, like psoriasis, but with clinical history.

  • Parapsoriasis: Poikiloderma (telangiectasia, atrophy) and/or band-like lymphocytic infiltrate.

2. Lichenoid Dermatitis

• Pattern: Band-like lymphocytic infiltrate at dermo-epidermal junction.

• Differential & Tie-Breaker (One Discriminator):

  • Lichen Planus: Saw-tooth rete ridges, civatte bodies (apoptotic keratinocytes).

  • Lichenoid Drug Eruption: Eosinophils and/or plasma cells in infiltrate.

  • Graft-vs-Host Disease: Satellite cell necrosis (lymphocyte adjacent to apoptotic keratinocyte).

  • Lichen Nitidus: Claw-like rete ridges clutching a focused lymphohistiocytic infiltrate.

  • Lupus Erythematosus: Interface vacuolization, dermal mucin, periadnexal and perivascular inflammation.

  • Pityriasis Lichenoides Chronica: Red blood cell extravasation into papillary dermis.

  • Lichen Sclerosus: Papillary dermal edema/homogenization below the lichenoid infiltrate.

  • Lichen Planopilaris: Lichenoid interface change focused on the follicular infundibulum.

  • Benign Lichenoid Keratosis: A "regressing" solar lentigo or seborrheic keratosis.

3. Vesiculobullous Disorders

• Pattern: Intraepidermal or subepidermal blisters.

• Differential & Tie-Breaker (One Discriminator):

  • Pemphigus Vulgaris: Suprabasal acantholysis (tombstone row of basal cells).

  • Pemphigus Foliaceus: Subcorneal acantholysis.

  • Bullous Pemphigoid: Eosinophil-rich subepidermal blister.

  • Dermatitis Herpetiformis: Neutrophilic microabscesses at dermal papillary tips.

  • Linear IgA Disease: Neutrophils along the basement membrane zone (subepidermal).

  • Epidermolysis Bullosa Acquisita: Subepidermal blister with millet-seed-like fibrin at base.

  • Porphyria Cutanea Tarda: "Festooned" dermal papillae in a cell-poor subepidermal blister.

  • Staphylococcal Scalded Skin Syndrome: Cleavage within the granular layer.

  • Toxic Epidermal Necrolysis: Full-thickness keratinocyte necrosis.

  • Incontinentia Pigmenti (Vesicular Stage): Spongiotic vesicles with numerous eosinophils.

4. Granulomatous Disorders

• Pattern: Focal collections of histiocytes.

• Differential & Tie-Breaker (One Discriminator):

  • Sarcoidosis: "Naked" granulomas (minimal surrounding lymphocytes).

  • Granuloma Annulare: Palisaded granuloma around degenerated collagen (necrobiosis) and mucin.

  • Necrobiosis Lipoidica: Broad, layered zones of necrobiosis in deep dermis/subcutis.

  • Foreign Body Granuloma: Foreign material within histiocytes (e.g., doubly refractive).

  • Tuberculosis: Caseating granulomas, positive AFB stain.

  • Lepromatous Leprosy: Foamy histiocytes packed with AFB (globi).

  • Rheumatoid Nodule: Central fibrinoid necrosis surrounded by palisading histiocytes.

  • Xanthogranuloma: Touton giant cells (lipid-laden with wreath of nuclei).

5. Vasculitic Disorders

• Pattern: Leukocytoclastic vasculitis (LCV): Neutrophils, nuclear dust, vessel wall damage.

• Differential & Tie-Breaker (One Discriminator):

  • IgA Vasculitis (Henoch-Schönlein): LCV with dominant IgA deposition on DIF.

  • Hypersensitivity Vasculitis: LCV with dominant IgG/IgM (immune complex).

  • Granulomatosis with Polyangiitis (GPA): Vasculitis with geographic necrosis and granulomas.

  • Erythema Elevatum Diutinum: LCV with marked dermal fibrosis (late stage).

  • Polyarteritis Nodosa (cutaneous): Necrotizing inflammation of medium-sized arteries in deep dermis/subcutis.

  • Urticarial Vasculitis: LCV with clinical wheals lasting >24h.

  • Septic Vasculitis (e.g., Meningococcemia): Numerous organisms within vessel walls.

6. Panniculitis

• Pattern: Inflammation of subcutaneous fat.

• Differential & Tie-Breaker (One Discriminator):

  • Erythema Nodosum: Septal panniculitis with Miescher's radial granulomas.

  • Erythema Induratum (Nodular Vasculitis): Lobular panniculitis with vasculitis and caseous necrosis.

  • Lupus Panniculitis: Lymphocytic lobular panniculitis with lymphoid follicles and dermal mucin.

  • Pancreatic Panniculitis: Lobular fat necrosis with "ghost-like" adipocytes and calcification.

  • α1-antitrypsin Deficiency Panniculitis: Necrotizing septal panniculitis with splaying of neutrophils between collagen fibers.

  • Sclerosing Panniculitis (Lipodermatosclerosis): Septal fibrosis, fat microcysts, and hemosiderin.

7. Pathognomonic Cells/Structures

• Langerhans Cell Histiocytosis: Birbeck granules (EM), CD1a+/Langerin+ cells.

• Sézary Syndrome: Cerebriform nuclei in CD4+ CD7- lymphocytes in blood.

• Verruca (HPV): Koilocytes (perinuclear halo, pyknotic nucleus).

• Juvenile Xanthogranuloma: Touton giant cells.

• Syphilis: Plasma cell-rich infiltrate, spirochetes on special stain.

• Arthropod Bite Reaction: Wedge-shaped, dense eosinophilic infiltrate.

• Bullous Pemphigoid: Eosinophil-rich subepidermal blister.

• Sweet's Syndrome: Dense dermal neutrophilic infiltrate without vasculitis.

• Mycosis Fungoides: Epidermotropic, atypical lymphocytes with cerebriform nuclei.

• Dermatofibroma: Collagen trapping at periphery creating a "cleft."

8. Special Stain Associations

• Fungi/Yeast (PAS/GMS): Dermatophytosis (hyphae in stratum corneum).

• Acid-Fast Bacilli (Fite): Lepromatous leprosy (globi in foamy histiocytes).

• Amyloid (Congo Red): Lichen amyloidosis (globular deposits in papillary dermis).

• DIF - Pemphigus: Fishnet pattern of intercellular IgG.

• DIF - Bullous Pemphigoid: Linear IgG/C3 at basement membrane.

• DIF - Dermatitis Herpetiformis: Granular IgA in dermal papillary tips.

• DIF - Lupus: Granular IgG/IgM/C3 at basement membrane (lupus band).

• DIF - IgA Vasculitis: IgA in vessel walls.

• Iron (Prussian Blue): Hemochromatosis (iron in basement membrane of sweat glands).

• Melanin (Fontana-Masson): Melanoma (atypical melanocytes with pigment).

• Mucin (Alcian Blue): Lupus erythematosus (increased dermal mucin).

• Calcium (Von Kossa): Calcinosis cutis (purple-black deposits).

• Lipid (Oil Red O - frozen): Xanthoma (foamy histiocytes).

• Spirochetes (Warthin-Starry): Syphilis (treponemes in epidermis/dermis).