Skin Rashes: Differential diagnosis by appearance

Pigmented lesions 

• Freckles (ephelides): flat, brown spots. 
• Lentigo: similar to freckles but darker and not affected by sunlight. 
• Seborrheic keratosis: benign, beige/brown plaques, 3-20 mm in diameter, with a velvety or warty surface. 
• Nevus: moles. 
• Blue nevus: small, slightly elevated, blue-black lesions. 
• Atypical (dysplastic) nevus: over 5 mm across, ill-defined, irregular border, irregularly distributed pigmentation with erythema and accentuated skin markings. 
• Melanoma: flat or raised pigmented lesion with possibly a recent change in appearance. It has varying colors and typically irregular borders. 

Scaly lesions 

• Psoriasis: silvery, scaled, well-demarcated plaques on skin, usually over the extensor surfaces. It can be pustular and involve the nails (nail pitting). 
• Dermatitis and eczema: a pruritic, exudative, or lichenified eruption on the face, neck, upper trunk, wrists, and hands, and in the antecubital and popliteal fossae. 
• Xerosis: dry skin. 
• Lichen simplex chronicus: chronic itching associated with pigmented, lichenified, skin lesions. Lichenified lesions exhibit exaggerated skin lines overlying thickened, well-circumscribed, scaly plaques. 
• Tinea corporis: ring-shaped lesion with an advancing scaly border and central clearing or scaly patches with a distinct border. 
• Tinea versicolor: pale or hyperpigmented macules, or velvety, tan, pink, whitish, or brown macules that scale with scraping. 
• Secondary syphilis. 
• Pityriasis rosea: oval, fawn-colored, scaly eruption following the cleavage lines of the trunk ("Christmas tree" pattern). It is commonly preceded by a herald patch. 
• Discoid lupus erythematosus: red, asymptomatic, localized plaques, usually on the face and often in a "butterfly" distribution. There is scaling, follicular plugging, atrophy, and telangiectasia of involved areas. 
• Exfoliative dermatitis: scaling and erythema over a large area of the body. 
• Actinic keratoses: small, pink patches that feel like bits off sandpaper when the finger is drawn over them. They are premalignant. 
• Bowen's disease (intraepidermal squamous cell carcinoma): small, well-demarcated, slightly raised, pink-to-red, scaly plaques. 
• Extramammary Paget's disease: this resembles chronic eczema and may involve apocrine areas such as the genitals. 
• Intertrigo: fissuring, erythema, and sodden epidermis, with superficial denudation in the body folds. 


Vesicular lesions 

• Herpes simplex: recurrent, small, grouped vesicles on an erythematous base, especially around the oral and genital areas; often described as "dew drops on a rose petal." 
• Herpes zoster: vesicular lesions in a dermatomal distribution. 
• Pompholyx: pruritic "tapioca" vesicles or bullae on the palms, soles, and sides of fingers. 
• Dermatophytid (allergy or sensitivity to fungi): pruritic, grouped, vesicular lesions involving the sides and flexor aspects of the fingers and palms. 
• Dermatitis herpetiformis: pruritic papulovesicular lesions, mainly on the elbows, knees, buttocks, posterior neck, and scalp. It is associated with gluten-sensitive (i.e., sprue) enteropathy. 
• Miliaria (heat rash): superficial, aggregated, small vesicles, papules, or pustules on covered areas of the skin. 
• Scabies: pruritic vesicles and pustules especially on the sides of the fingers. Red papules or nodules on the penile glans and shaft are pathognomonic. 
• Photosensitivity. 

Weepy or encrusted lesions 

• Impetigo: vesiculopustular lesions with thick, golden-crusted exudate associated with group A β-hemolytic streptococci or coagulase-positive Staphylococcus aureus, or bullous lesions associated with S. aureus. 
• Acute-contact allergic dermatitis: erythema and oedema, with pruritus, often followed by vesicles and bullae in an area of contact with a suspected agent. They may later weep, crust, and become infected. 
• Any vesicular dermatitis. 

Pustular lesions 

Any type of superficial skin infection (bacterial or fungal) can cause pustules. 
• Acne vulgaris: the most common skin condition, characterized by open and closed comedones. It varies from purely comedonal to pustular inflammatory acne to cysts to nodules. 
• Acne rosacea: erythema and telangiectasia and a tendency to flush easily. May have an acneiform component, or hyperplasia of the soft tissue of the nose (rhinophyma). 
• Folliculitis: pustules in the hair follicles. 
• Candidiasis: superficial, denuded, beefy-red areas with or without satellite vesicopustules. Whitish, curd-like concretions on the oral and vaginal mucous membranes. 
• Miliaria. 
• Any vesicular dermatitis. 

Figurate erythema 

Figurate erythema lesions look like rings or arcs. 
• Urticaria: eruptions of evanescent wheals or hives. 
• Erythema multiforme: symmetrical erythematous lesions on the extensor surfaces, palms, soles, or mucous membranes, which may be macular, papular, urticarial, bullous, or purpuric. Target lesions with clear centers and concentric erythematous rings may be present. This is often associated with drug rashes. 
• Erythema migrans: a red expansion around an initial papule with an advancing border, which is usually raised, warm, and red. The center may clear or become indurated, vesicular, or necrotic. This is commonly associated with Lyme disease. 
• Cellulitis: a hot, red, diffuse, spreading infection of the skin. 
• Erysipelas: edematous, spreading, circumscribed, hot, erythematous area, with or without vesicle or bulla formation, frequently involving the face. 
• Erysipeloid: purplish erythema, most often of a finger or the back of the hand, which gradually extends. Caused by Erysipelothrix insidiosa, it is often seen in fishermen and meat handlers. 
Bullous lesions 
• Impetigo: superficial bacterial infection caused by group A β-hemolytic streptococci or S. aureus. 
• Pemphigus: relapsing crops of bullae appearing on normal skin, often preceded by mucous membrane bullae, erosions, and ulcerations. There may be superficial detachment of the skin after pressure or trauma (Nikolsky's sign). 
• Bullous pemphigoid: tense blisters in flexural areas. They may be preceded by urticarial or edematous lesions. 
• Porphyria cutanea tarda. 
• Erythema multiforme: "target" lesions, i.e., symmetrically distributed, circular lesions, often with a central blister. 
• Toxic epidermal necrolysis: usually secondary to drugs (e.g., sulphonamides, penicillins, and anticonvulsants). This is a severe form of Steven-Johnson syndrome. 

Papular lesions 

• Hyperkeratotic: warts, corns, seborrheic keratoses. 
• Purple: lichen planus-pruritic, violaceous, flat-topped papules with fine white streaks and a symmetrical distribution, commonly seen along linear scratch marks on the anterior wrists, sacral region, penis, legs, and mucous membranes; drug eruptions; Kaposi's sarcoma-malignant skin lesions with dark plaques or nodules on cutaneous or mucosal surfaces, common in people with human immunodeficiency virus infection. 
• Flesh-colored and umbilicated: molluscum contagiosum-a viral infection causing single or multiple, rounded, dome-shaped, waxy papules, 2-5 mm in diameter, which are umbilicated and contain a caseous plug. 
• Pearly: basal cell carcinoma-most commonly papules or nodules with a central scab or erosion; intradermal nevi. 
• Small, red, and inflammatory: acne, miliaria, candidiasis, intertrigo, scabies, folliculitis
Nodular, cystic lesions 
• Erythema nodosum: painful red nodules without ulceration on the anterior aspects of the legs; they may regress over weeks to resemble contusions. This is associated with inflammatory bowel disease. 
• Furuncle (boils): painful inflammatory swellings of a hair follicle forming an abscess, caused by S. aureus. 
• Cystic acne. 
• Follicular (epidermal) inclusion cyst. 

Photodermatoses 
Painful erythema, edema, and vesiculation on sun-exposed surfaces, usually the face, neck, hands, and V of the chest. Causes include drugs (e.g., amiodarone, phenothiazines, sulphonamides, and related drugs), polymorphic light eruption, and systemic lupus erythematosus (SLE). 


Maculopapular lesions 

Viral causes and secondary syphilis. 

Erosive lesions 

• Any vesicular dermatitis. 
• Impetigo. 
• Lichen planus. 
• Erythema multiforme. 
• Oral erosions. 
• Psychiatric disorders. 
Ulcerated lesions 
• Decubiti: bed sores or pressure sores. 
• Herpes simplex. 
• Skin cancers. 
• Parasitic infections. 
• Syphilis: chancre. 
• Vasculitis. 
• Stasis. 
• Arterial disease.