give me every possible ddx for final exam of deramtology topics hand eczema, icd, acd, acne vulgaris, acne rosacea, perioral dermatitis, psoriasis, seborrheic dermatitis, pityriasis rosea, lichen planus, impetigo, folliculitis, carbuncle, furuncle, all tinea inf., candidiasis, syphilis, hsv, hzv, warts, molluscum contagiosum, scabies, pediculosis, , pemphigus vulgaris, bullous pemphigoid, alopecia areata, androgenic alopecia, vitiligo, melanoma, seborrheic keratosis, bcc, scc these all tell each of them

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Now let me search the dermatology library for all these topics in parallel batches.

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Now let me get the detailed content from key blocks for the most important topics:

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Finding Sources

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Finding Sources

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I now have comprehensive data from the textbooks. Let me compile the complete DDx guide.Here is your complete, exam-ready DDx guide for every topic listed, sourced directly from Fitzpatrick's, Dermatology 2-Volume Set 5e, and Andrews' Diseases of the Skin.

Complete Dermatology DDx - Final Exam Master Guide

ECZEMATOUS / INFLAMMATORY DERMATOSES

1. Hand Eczema (Dyshidrotic / Vesiculobullous / Hyperkeratotic)

Core DDx:

Psoriasis - most important (sharply demarcated plaques, nail pitting, pustule crops are clues to psoriatic hand disease)
Tinea manuum - check KOH; often unilateral ("two feet, one hand" pattern)
Allergic Contact Dermatitis (ACD) - patch testing differentiates
Irritant Contact Dermatitis (ICD) - distribution matches exposure
Scabies - interdigital burrows, nocturnal itch, household spread
Calluses / Warts - hyperkeratotic variant
Porokeratosis
Secondary syphilis (palmoplantar involvement)
Pompholyx / Dyshidrotic eczema - when primarily vesicular

Key clue: For pulpitis (fingertip eczema) of dominant hand - think ACD; nondominant hand - think food handling (garlic, vegetables).

Andrews' Diseases of the Skin, p. 95

2. Irritant Contact Dermatitis (ICD)

Core DDx:

Allergic Contact Dermatitis (ACD) - most critical distinction (patch test positive in ACD; history of delay 48-72h)
Contact urticaria - immediate reaction, resolves within hours
Atopic dermatitis - chronic, pruritic, flexural distribution, family history of atopy
Nummular eczema - coin-shaped plaques
Seborrheic dermatitis - if facial distribution
Psoriasis - if hands/scalp involved
Tinea - KOH positive
Intertrigo - intertriginous variant; DDx includes candidiasis, seborrheic dermatitis, inverse psoriasis, erythrasma
Dermatology 2-Volume Set 5e, p. 283

3. Allergic Contact Dermatitis (ACD)

Core DDx:

Irritant Contact Dermatitis - no sensitization period, patch test negative
Atopic dermatitis - chronic, flexural, begins in childhood
Nummular eczema - coin-shaped, not distribution-dependent
Seborrheic dermatitis - scalp/face/chest; greasy scale
Psoriasis - silvery scale, nail findings
Phototoxic/photoallergic reaction - sun-exposed distribution
Tinea corporis/faciei - KOH positive, centrifugally expanding ring
Perioral dermatitis - if facial ACD mimics it
Scabies - if intensely pruritic with burrows
Fitzpatrick's Dermatology, Vol. 1-2

4. Acne Vulgaris

Core DDx:

Rosacea - no comedones, centrofacial, telangiectasias, flushing, older age
Perioral dermatitis - perioral/perinasal/periorbital sparing zone around lips
Folliculitis (staphylococcal) - monomorphous pustules, no comedones
Gram-negative folliculitis - complicates antibiotic-treated acne; central face
Malassezia (Pityrosporum) folliculitis - fine papulopustules on forehead/upper trunk
Pseudofolliculitis barbae - Afro-textured hair, beard area
Acne keloidalis nuchae - posterior neck
Eosinophilic folliculitis - HIV setting, markedly pruritic
Milia - white non-inflammatory cysts
Sebaceous hyperplasia - yellowish lobulated papules, no pustules
Steroid folliculitis - monomorphous papulopustules after oral corticosteroids, trunk
Keratosis pilaris - follicular keratotic papules, arms/thighs
Angiofibromas (tuberous sclerosis) - nasolabial folds, no inflammatory phase
Nevus comedonicus - linear array, congenital
Dermatology 2-Volume Set 5e, p. 636

5. Acne Rosacea

Core DDx:

Acne vulgaris - comedones present, younger age, no flushing, no phymatous changes
Seborrheic dermatitis - yellowish-orange greasy scale, nasolabial fold distribution
Systemic Lupus Erythematosus (SLE) - butterfly rash spares nasolabial folds, photosensitive, ANA positive
Discoid Lupus Erythematosus (CDLE) - scarring, follicular plugging, atrophy
Perioral dermatitis - perioral/perinasal, unaffected zone around vermilion border
Contact dermatitis - pruritic, eczematous morphology, history of exposure
Polymorphic light eruption - photodistributed, more monomorphic
Dermatomyositis - heliotrope rash, Gottron papules, muscle weakness
Sarcoidosis - non-caseating granulomas on biopsy, ACE elevated
Demodicosis - Demodex mite infestation, diffuse follicular papulopustules
Erysipelas - acute onset, fever, sharp demarcation, leukocytosis
Carcinoid flush - episodic flushing, elevated 5-HIAA
Fitzpatrick's Dermatology, Vol. 1-2

6. Perioral Dermatitis (Periorificial Dermatitis)

Core DDx:

Acne vulgaris - comedones present, no perioral sparing
Rosacea - flushing, telangiectasias, phymas; can coexist
Allergic contact dermatitis - history of topical exposure, eczematous morphology
Irritant contact dermatitis - related to cosmetics, topicals
Demodicosis - Demodex infestation
Lupus miliaris disseminatus faciei - small discrete papules, caseating granulomas on biopsy
Seborrheic dermatitis - scalp/face/chest involvement
Molluscum contagiosum - umbilicated papules
Tinea faciei - KOH positive, annular
Systemic LE - photodistributed, ANA
Steroid-induced rosacea - history of fluorinated topical steroid use
Fitzpatrick's Dermatology, Vol. 1-2

PAPULOSQUAMOUS DISORDERS

7. Psoriasis (Plaque / Guttate / Pustular)

Plaque Psoriasis DDx:

Seborrheic dermatitis ("sebopsoriasis") - less thick scale, greasy, scalp/face/chest
Lichen planus - violaceous, Wickham striae, mucosal involvement
Tinea corporis - KOH positive, centrifugal spread
Nummular eczema - small coin-shaped, vesicles
Hypertrophic lichen planus - shins, biopsy differentiates
Secondary syphilis - palms/soles involvement, RPR positive
Cutaneous T-cell lymphoma (CTCL/mycosis fungoides) - chronic, atypical histology
Pityriasis rubra pilaris - follicular hyperkeratosis, islands of normal skin

Guttate Psoriasis DDx:

Pityriasis rosea - herald patch, collarette scale, Christmas tree pattern
Secondary syphilis - palms/soles, RPR
Pityriasis lichenoides chronica - no herald patch, more persistent
Small plaque parapsoriasis
Drug eruption

Pustular Psoriasis DDx:

AGEP (acute generalized exanthematous pustulosis) - drug reaction, acute febrile onset, not recurrent
Sneddon-Wilkinson disease (subcorneal pustular dermatosis)
DIRA / DITRA - autoinflammatory
Dermatophyte infection - for palmoplantar variant
Candidiasis

Psoriasis Inversa (Flexural) DDx:

Seborrheic dermatitis, contact dermatitis, cutaneous candidiasis, tinea incognito, erythrasma, extramammary Paget disease
Dermatology 2-Volume Set 5e, p. 174

8. Seborrheic Dermatitis

Core DDx:

Psoriasis ("sebopsoriasis") - thicker silvery scale, more discrete, less pruritic; nail pitting
Atopic dermatitis - dry brittle hair, dry skin, chronic flexural involvement
Tinea capitis - KOH/culture positive (scalp)
SLE butterfly rash - spares nasolabial folds, photodistributed, ANA positive
Rosacea - frequently coexists; telangiectasias, flushing
Pityriasis rosea - trunk lesions are ellipsoid with collarette scale, no facial predilection
Subacute cutaneous LE - annular/polycyclic lesions, anti-Ro antibody
Dupilumab-associated facial dermatitis - history of biologic use
Intertrigo (seborrheic) - DDx includes inverse psoriasis, candidiasis, erythrasma, Langerhans cell histiocytosis
Dermatology 2-Volume Set 5e, p. 283

9. Pityriasis Rosea

Always Rule Out:

Secondary syphilis - most important; involves palms/soles, no herald patch, RPR positive, condyloma lata, lymphadenopathy

Most Likely:

Tinea corporis - KOH positive; scale at periphery not center; not oval or along Langer lines
Nummular eczema - circular (not oval), vesicles, no collarette

Consider:

Guttate psoriasis - smaller lesions, thick silvery scale, not along skin lines
Pityriasis lichenoides chronica - no herald patch, more chronic, extremity-dominant
Lichen planus - more pruritic, chronic, distal extremity, Wickham striae
Seborrheic dermatitis - face/scalp involvement
Drug eruption - medication history
Parapsoriasis
Viral exanthem
Fitzpatrick's Dermatology, Vol. 1-2 (Table 31-1)

10. Lichen Planus

Core DDx:

Psoriasis - shins; hypertrophic LP mimics plaque psoriasis; biopsy differentiates
Lichenoid drug eruption - drug history; photodistributed; eosinophils on biopsy
Secondary syphilis - RPR positive; palms/soles
Discoid LE - scarring, follicular plugging, photodistributed
Graft-vs-host disease (GVHD) - stem cell transplant history
Lichen simplex chronicus - repetitive rubbing; no Wickham striae
Nummular eczema
Pityriasis rosea - self-limited, herald patch
Lichen sclerosus - atrophic LP variant at genital/mucosal sites
Mycosis fungoides (MF) - atrophic LP variant; biopsy essential
Bullous LP vs. Bullous pemphigoid - immunofluorescence differentiates
Oral LP DDx: aphthous ulcers, candidiasis, SLE, pemphigus vulgaris, cicatricial pemphigoid
Fitzpatrick's Dermatology, Vol. 1-2

BACTERIAL INFECTIONS

11. Impetigo

Non-Bullous Impetigo DDx:

Most common: insect bites, eczematous dermatoses, herpes simplex, candidiasis
Less common: inflammatory tinea corporis/faciei, varicella, scabies, pediculosis, pemphigus foliaceus

Bullous Impetigo DDx:

Most common: bullous insect bite reactions, thermal burns, herpes simplex, acute contact dermatitis (ACD or ICD)
Less common: autoimmune bullous dermatoses, bullous erythema multiforme, Stevens-Johnson syndrome, bullous mastocytosis
Dermatology 2-Volume Set 5e (Table 74.2)

12. Folliculitis

Core DDx:

Acne vulgaris - comedones present
Furunculosis - deeper, more painful nodular lesion
Pseudofolliculitis barbae - ingrown hairs, beard area
Tinea barbae / Kerion - KOH positive
Malassezia folliculitis - upper trunk, forehead; no comedones; responds to antifungals
Gram-negative folliculitis - complicates oral antibiotic therapy
Pseudomonas ("hot tub") folliculitis - bathing suit distribution
Eosinophilic folliculitis - HIV, pruritic
Demodex folliculorum folliculitis - facial, older adults
Milia - non-inflammatory
Rosacea - central face, telangiectasias, flushing
Keratosis pilaris - non-inflammatory keratotic plugs

13. Furuncle (Boil)

Core DDx:

Carbuncle - multiple follicles, deeper, multiple drainage points
Hidradenitis suppurativa (HS) - recurrent, axilla/groin/inframammary, sinus tracts
Epidermoid (sebaceous) cyst - no surrounding erythema unless ruptured/infected
Nodular acne - acne-prone areas, comedones present
Abscess - non-follicular origin
Anthrax - eschar, exposure history
Sporotrichosis - lymphocutaneous spread

14. Carbuncle

Core DDx:

Furuncle - single follicle involvement only
Hidradenitis suppurativa - recurrent, apocrine-bearing areas
Deep fungal infection (e.g., blastomycosis) - endemic exposure
Actinomycosis - jaw, sinus tract with sulfur granules
Nodular lymphoma / cutaneous metastasis - non-infectious; does not drain

FUNGAL INFECTIONS

15. All Tinea Infections

Tinea	Core DDx
Tinea corporis	Nummular eczema, atopic/contact/seborrheic dermatitis, psoriasis, pityriasis rosea, erythema annulare centrifugum, granuloma annulare, subacute cutaneous LE, tinea versicolor, parapsoriasis, impetigo, secondary syphilis, mycosis fungoides
Tinea cruris	Cutaneous candidiasis (less sharply demarcated), intertrigo, seborrheic dermatitis, psoriasis (inverse), erythrasma, contact dermatitis, lichen simplex chronicus, Hailey-Hailey disease, extramammary Paget disease
Tinea pedis	Dyshidrotic eczema, contact dermatitis, psoriasis (pustular/vulgaris), juvenile plantar dermatosis, secondary syphilis, erythrasma (interdigital type), bacterial infection (Gram-negative rods)
Tinea capitis	Seborrheic dermatitis, alopecia areata, trichotillomania, psoriasis, pyoderma/folliculitis (if pustules), lichen planus/DLE/folliculitis decalvans (if scarring)
Tinea faciei	Seborrheic dermatitis, perioral dermatitis, contact dermatitis, rosacea, lupus erythematosus, acne vulgaris, annular psoriasis
Tinea unguium (onychomycosis)	Psoriasis (most common), lichen planus, chronic trauma, onychogryphosis, Darier disease, yellow nail syndrome; Rule out: melanoma
Tinea versicolor	Vitiligo (depigmented macules), seborrheic dermatitis, pityriasis alba, pityriasis rosea, progressive macular hypomelanosis, pityriasis lichenoides, dyschromatosis universalis hereditaria

Dermatology 2-Volume Set 5e (Table 77.9)

16. Candidiasis (Cutaneous)

Core DDx:

Intertrigo - less bright red, no satellite lesions
Tinea cruris - more sharply demarcated; KOH shows hyphae (not pseudohyphae/yeast)
Contact dermatitis (ICD or ACD) - history of exposure
Inverse psoriasis - well-demarcated, silvery scale elsewhere
Seborrheic dermatitis - greasy scale, scalp/face
Erythrasma - coral-red fluorescence on Wood lamp (Corynebacterium)
Hailey-Hailey disease - acantholysis on biopsy, familial
Herpes simplex (herpetic whitlow) - if finger involved
Folliculitis - if follicular pustules present
Rosen's Emergency Medicine

SEXUALLY TRANSMITTED / VIRAL INFECTIONS

17. Syphilis (Secondary)

Core DDx:

Pityriasis rosea - herald patch, no palms/soles, RPR negative
Pityriasis lichenoides - more chronic, RPR negative
Guttate psoriasis - thick silvery scale, no palms/soles
Lichen planus - violaceous, Wickham striae, more pruritic
Drug eruption - medication history
Acute febrile viral exanthem - fever, viral prodrome
Tinea corporis - KOH positive
Reactive arthritis (Reiter syndrome) - keratoderma blennorrhagica, triad
Condyloma lata vs. genital warts (condyloma acuminata) - broad-based flat lesions, RPR positive
Goldman-Cecil Medicine

18. Herpes Simplex Virus (HSV)

Core DDx:

Herpes zoster - dermatomal distribution, older age, post-herpetic neuralgia; zosteriform HSV can mimic
Impetigo - honey-colored crusts, no recurrence pattern
Aphthous ulcers - no grouped vesicles, not viral
Varicella - generalized distribution, various stages simultaneously
Hand-foot-mouth disease (Coxsackievirus) - palms/soles/oral
Erythema multiforme - target lesions, may be triggered by HSV
Contact dermatitis - vesicles without grouped pattern
Fixed drug eruption - recurs at same site, no vesicles initially
Pemphigus vulgaris - oral lesions, Tzanck negative
Herpetic whitlow - DDx: acute bacterial paronychia, felon, whitlow
Fitzpatrick's Dermatology, Vol. 1-2

19. Herpes Zoster (HZV)

Core DDx:

Zosteriform herpes simplex - most important; recurrent, no dermatomal prodrome of zoster type; Tzanck/PCR differentiates
Contact dermatitis - no dermatomal pattern, no pain/burning prodrome
Varicella - generalized; children; various stages
Impetigo - no dermatomal distribution
Insect bites - no dermatomal pattern
Pre-eruptive zoster - before rash, mimics:
- MI (chest dermatome)
- Appendicitis / renal colic (abdominal dermatome)
- Sciatica / herniated disc (lumbar/sacral dermatome)
- Otitis externa / Ramsay Hunt (auricular)
Tintinalli's Emergency Medicine

20. Warts (Verruca Vulgaris / Flat / Plantar)

Common Warts (Verruca vulgaris) DDx:

Seborrheic keratosis - stuck-on appearance, no pinpoint bleeding
Molluscum contagiosum - umbilicated, smooth
Callus / corn - no black dots on paring; located over pressure points
Squamous cell carcinoma - especially if periungual or on immunocompromised patient

Flat Warts (Verruca plana) DDx:

Lichen planus (Wickham striae), lichen nitidus, acrokeratosis verruciformis, Gottron papules of dermatomyositis (over knuckles), lichenoid keratoses

Plantar Warts DDx:

Corn (heloma durum) - central hard core, no black dots
Callus - no interruption of dermatoglyphics
Secondary syphilis - palmoplantar papulosquamous lesions
Amelanotic melanoma - important to exclude

Genital Warts (Condyloma acuminata) DDx:

Condyloma lata (secondary syphilis) - flat, moist; RPR positive
Molluscum contagiosum, skin tags (fibroepithelial polyps), normal anatomic variants (pearly penile papules), nevi, SCC
Rosen's Emergency Medicine

21. Molluscum Contagiosum

Core DDx:

Warts - no umbilication; HPV
Milia - no umbilication, not infectious
Closed comedones (acne) - no umbilication
Basal cell carcinoma (nodular) - pearly translucent nodule; biopsy needed
Sebaceous hyperplasia - yellowish, lobulated, no umbilication
Cryptococcosis - in immunocompromised; can mimic molluscum
Histoplasmosis - disseminated; HIV setting
Orf / milker's nodule - occupational exposure; cattle
Keratoacanthoma - crater-like, larger
Condyloma acuminata - genital; HPV DNA testing

ECTOPARASITES

22. Scabies

Core DDx (Fitzpatrick's Table 178-1): Most Likely:

Atopic dermatitis
Dyshidrotic eczema
Pyoderma
Contact dermatitis
Insect bite reaction

Also consider:

Pediculosis
Bullous pemphigoid (scabies can trigger it)
Drug eruption
Urticaria / dermatographism
Lichen planus
Viral exanthem
Delusions of parasitosis
Dermatitis herpetiformis

Key clue: burrows in finger webs, wrists, genitalia; nocturnal pruritus; household spread.

23. Pediculosis (Head / Body / Pubic Lice)

Head Lice DDx (Fitzpatrick's Table 178-4):

Seborrheic dermatitis (dandruff) - scales not attached to hair shaft
Insect bites
Eczema / atopic dermatitis
Psoriasis
Hair gel / hairspray artifact
Tinea capitis

Body Lice DDx:

Systemic diseases causing pruritus (hepatic/renal impairment)
Drug reactions
Atopic dermatitis, contact dermatitis
Scabies
Other infestations

Pubic (Crab) Lice DDx (Fitzpatrick's Table 178-7):

Excoriations
Scabies
Contact dermatitis
Piedra
Trichomycosis pubis
Hair casts
Nevi

BULLOUS DISORDERS

24. Pemphigus Vulgaris

Oral Mucosal Lesions DDx:

Aphthous stomatitis - no skin lesions, no autoantibodies
Acute herpetic stomatitis (HSV) - grouped vesicles, Tzanck positive
Erythema multiforme / Stevens-Johnson syndrome - target lesions, acute onset
Lichen planus (erosive) - Wickham striae, DIF negative
Systemic LE - anti-dsDNA, photosensitive
Mucous membrane pemphigoid (cicatricial) - predominantly mucosal scarring; subepidermal on DIF
Paraneoplastic pemphigus - underlying malignancy; polymorphous eruption

Cutaneous Lesions DDx:

Bullous pemphigoid - tense blisters (subepidermal); anti-BP180/BP230 antibodies; older patients
Linear IgA bullous dermatosis - IgA on DIF
Hailey-Hailey disease - "dilapidated brick wall" acantholysis; no autoantibodies; familial
Transient acantholytic dermatosis (Grover disease) - small foci; older men; dyskeratosis
Erythema multiforme
Contact dermatitis (bullous)
Dermatology 2-Volume Set 5e, p. 620

25. Bullous Pemphigoid

Core DDx:

Pemphigus vulgaris - flaccid blisters (intraepidermal); positive Nikolsky sign; anti-Dsg3/Dsg1 antibodies
Dermatitis herpetiformis - IgA deposits; gluten-sensitive enteropathy; extremely pruritic; grouped vesicles on extensor surfaces
Linear IgA disease - linear IgA on DIF; "string of pearls" vesicles
Epidermolysis bullosa acquisita (EBA) - trauma-induced blisters; anti-collagen VII; salt-split DIF differentiates
Cicatricial pemphigoid - predominantly mucosal; scarring; head/neck skin blisters
Contact dermatitis (bullous) - history of allergen/irritant
Dyshidrotic eczema - small hands/feet vesicles
Bullous insect bite reaction
Stasis dermatitis with bullae - lower legs; chronic venous insufficiency
Bullous scabies - burrows present
Fitzpatrick's Dermatology, Vol. 1-2 (Table 54-1)

HAIR DISORDERS

26. Alopecia Areata

Core DDx:

Tinea capitis - KOH/culture positive; scaling; broken hairs; more common in children
Trichotillomania - no exclamation point hairs; geometric/irregular patches; evidence of pulling; no scaling
Traction alopecia - hairline thinning from hairstyle tension
Temporal triangular alopecia - congenital; triangular patch
Androgenetic alopecia (AGA) - diffuse variant of AA can mimic; trichoscopy differentiates
Telogen effluvium - diffuse; no well-defined patches; preceded by stressor
Secondary syphilis - "moth-eaten" alopecia; RPR positive
Pressure alopecia - ICU/surgical positioning
Aplasia cutis - congenital absence of skin
"Burnt-out" cicatricial alopecia - end-stage; biopsy needed
Dermatology 2-Volume Set 5e, p. Ch. 69

27. Androgenetic Alopecia (AGA)

Core DDx:

Telogen effluvium (TE) - acute diffuse shedding; precipitating event 2-3 months prior; recovers spontaneously; trichoscopy different
Diffuse alopecia areata - can mimic AGA; look for exclamation point hairs; trichoscopy
Hypothyroidism / hyperthyroidism - diffuse hair loss; thyroid labs
Iron deficiency anemia - serum ferritin low
Anagen effluvium - chemotherapy; rapid onset
Trichotillomania - irregular patches, evidence of pulling
Secondary syphilis (alopecia areolaris) - RPR positive
Chronic cutaneous lupus - scarring component; biopsy
Fitzpatrick's Dermatology, Vol. 1-2

PIGMENTARY DISORDERS

28. Vitiligo

Core DDx (Fitzpatrick's Tables 76-1 & 76-2):

Chemical/drug-induced leukoderma - cytotoxic to melanocytes; occupational phenol/catechol exposure; history
Postinflammatory hypopigmentation - follows preceding inflammatory dermatosis
Pityriasis (tinea) versicolor - KOH positive; scale present
Pityriasis alba - ill-defined, mild scale; children; atopic background
Nevus depigmentosus - present from birth; does not spread; no melanocyte loss on biopsy
Piebaldism - congenital; white forelock; stable; KIT mutation
Lichen sclerosus (mucosal vitiligo) - extragenital LS vs. mucosal vitiligo
Idiopathic guttate hypomelanosis - sun-exposed; older adults; small macules
Progressive macular hypomelanosis - trunk; no fluorescence; no scale
Leprosy - anesthetic patches; thickened nerves
Albinism - generalized, congenital
Halo nevus - depigmented ring around melanocytic nevus
Harrison's Principles of Internal Medicine 22E

SKIN TUMORS

29. Melanoma

Core DDx (use ABCDE criteria to differentiate):

Dysplastic (atypical) nevus - most challenging; dermoscopy, biopsy
Blue nevus - uniform blue color, no asymmetry
Pigmented Spitz nevus - young patients; red-brown; dermoscopy
Pigmented spindle cell nevus (Reed nevus) - adolescent females; dark uniform
Pigmented BCC - pearly rolled border; arborizing vessels on dermoscopy
Seborrheic keratosis (black SK) - "stuck on," milia cysts, comedo-like openings on dermoscopy
Dermatofibroma (hemosiderotic variant) - central scar-like on dermoscopy
Vascular tumor (hemangioma, pyogenic granuloma) - red/violaceous; lacunae on dermoscopy
Acral lentiginous melanoma vs. Subungual hematoma - Hutchinson sign; nail clipping
Amelanotic melanoma - can mimic many lesions (BCC, SCC, granuloma); biopsy essential
Lentigo maligna (melanoma in situ) - sun-damaged skin; solar lentigo DDx
Dermatology 2-Volume Set 5e, p. Ch. 113

30. Seborrheic Keratosis (SK)

Core DDx:

Melanoma - most important; solitary black SK vs. melanoma (smooth, infiltrating surface in melanoma; stuck-on, warty in SK)
Pigmented BCC - arborizing vessels on dermoscopy
Actinic keratosis - rough, scaly, premalignant; sun-damaged skin
Wart (verruca vulgaris) - pits black dots (thrombosed capillaries); HPV
Condyloma acuminata - genital
Epidermal nevus - present from birth; linear arrangement
Solar lentigo - flat, macular variant; macular SK
Dermatofibroma - dimple sign; firm
Andrews' Diseases of the Skin

31. Basal Cell Carcinoma (BCC)

By Subtype:

Nodular BCC DDx:

Intradermal melanocytic nevus, trichoepithelioma, trichoblastoma, sebaceous hyperplasia, adnexal tumors (syringoma, etc.), molluscum contagiosum, amelanotic melanoma, pigmented SK

Superficial BCC DDx:

SCC in situ (Bowen disease), nummular dermatitis/eczema, psoriasis, seborrheic dermatitis, lichenoid keratosis, actinic keratosis
Key clue: isolated "eczema" patch not responding to treatment = biopsy to rule out superficial BCC

Morpheaform BCC DDx:

Scar / morphea - no previous trauma? = biopsy
Microcystic adnexal carcinoma (MAC) - CK15 and CEA staining help

Pigmented BCC DDx:

Melanoma, blue nevus, pigmented SK, dermatofibroma
Fitzpatrick's Dermatology, Vol. 1-2

32. Squamous Cell Carcinoma (SCC)

Core DDx:

Keratoacanthoma (KA) - rapid growth over weeks, central keratin crater; spontaneous regression possible; many consider it low-grade SCC
Actinic keratosis (AK) - precursor; flat/slightly elevated, rough; no induration
Bowen disease (SCC in situ) - erythematous plaque; HPV-associated variants
Hypertrophic lichen planus - shins; biopsy
Verruca vulgaris - black dots; no induration; younger patients
Amelanotic melanoma - critical to exclude; biopsy
BCC - pearly rolled border; different histology
Prurigo nodularis - excoriated nodules
Deep fungal infection (blastomycosis, chromomycosis) - verrucous plaque; exposure history
Merkel cell carcinoma - flesh-colored rapidly growing nodule; older immunocompromised
Cutaneous metastasis - firm dermal/subcutaneous nodule; known malignancy
Dermatology 2-Volume Set 5e, p. Ch. 108

QUICK CROSS-REFERENCE TABLE

Presentation	Top DDx to Exclude
Palmoplantar papulosquamous lesions	Secondary syphilis (always RPR)
Unilateral annular scaling plaque	Tinea corporis (KOH first)
Moth-eaten alopecia	Secondary syphilis
Herald patch + Christmas tree rash	Pityriasis rosea vs. secondary syphilis
Tense blisters, elderly	Bullous pemphigoid
Flaccid blisters + oral erosions	Pemphigus vulgaris
Dermatomal vesicles	HZV vs. zosteriform HSV
Honey-crusted face lesion	Impetigo vs. HSV
Non-healing "eczema" patch	Superficial BCC, CTCL
Hyperkeratotic fissured hands	Psoriasis vs. hand eczema
Umbilicated papules + HIV	Cryptococcosis vs. molluscum
Follicular pustules without comedones	Folliculitis vs. rosacea vs. malassezia
White depigmented patches	Vitiligo vs. pityriasis versicolor vs. pityriasis alba
Black stuck-on plaques	SK vs. melanoma

Sources: Fitzpatrick's Dermatology Eds. Vol. 1-2 | Dermatology 2-Volume Set 5e (Bolognia) | Andrews' Diseases of the Skin | Goldman-Cecil Medicine | Tintinalli's Emergency Medicine | Rosen's Emergency MedicineAll 32 conditions are covered. Here are a few high-yield exam tips to go with the guide above:

High-Yield Exam Tips

The "Always Rule Out" Principle:

Any papulosquamous eruption involving palms/soles = RPR for secondary syphilis
Any non-healing "eczema" plaque = biopsy for superficial BCC or CTCL
Any single dark lesion on a nail = biopsy for subungual melanoma

The Key Distinguishing Tests:

Condition	Confirmatory Test
Tinea	KOH preparation
Candidiasis	KOH (pseudohyphae + yeast)
Scabies	Skin scraping + microscopy
Syphilis	RPR/VDRL then FTA-ABS
HSV/HZV	Tzanck smear, PCR
Pemphigus vulgaris	DIF (IgG intercellular)
Bullous pemphigoid	DIF (IgG linear at BMZ)
Alopecia areata	Trichoscopy ± biopsy
Melanoma	Dermoscopy + excisional biopsy

The "No Comedones" Rule:

Rosacea, perioral dermatitis, folliculitis, eosinophilic folliculitis = no comedones (unlike acne vulgaris)

Nikolsky Sign:

Positive = pemphigus vulgaris (intraepidermal split, flaccid blisters)
Negative = bullous pemphigoid (subepidermal split, tense blisters)

also give ddx of atopic dermatitis

Finding Sources

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Finding Sources

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