Dermatology · Papulosquamous Disorders
The facts most likely to be tested
Psoriasis is a T-cell mediated chronic inflammatory skin condition characterized by hyperproliferation of keratinocytes.
The classic physical exam finding is a well-demarcated, erythematous plaque with overlying silvery micaceous scale.
Auspitz sign is the appearance of punctate bleeding when the psoriatic scale is forcibly removed.
Koebner phenomenon describes the development of psoriatic lesions at sites of skin trauma or injury.
Guttate psoriasis typically presents as teardrop-shaped papules following a recent Group A Streptococcus pharyngitis infection.
Psoriatic arthritis is a seronegative spondyloarthropathy that classically presents with dactylitis (sausage digits) and pencil-in-cup deformity on radiographs.
First-line treatment for localized plaque psoriasis is high-potency topical corticosteroids combined with vitamin D analogs like calcipotriene.
Vignette unlocked
A 28-year-old male presents to the clinic with a persistent rash on his elbows and knees. Physical examination reveals well-demarcated erythematous plaques covered in a thick, silvery micaceous scale. When the physician scrapes the surface of the lesion, punctate bleeding is observed. The patient also reports morning stiffness in his fingers, which appear swollen and resemble a sausage digit.
What is the most likely diagnosis and the underlying pathophysiology?
Psoriasis and Psoriatic Arthritis; T-cell mediated hyperproliferation of keratinocytes.
The vignette describes classic plaque psoriasis (Auspitz sign) and associated psoriatic arthritis (dactylitis), which is driven by T-cell mediated immune dysregulation.
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High yield triage
Etiology / Epidemiology
Chronic immune-mediated disease with bimodal distribution (20s and 50s) and genetic predisposition (HLA-Cw6).
Clinical Manifestations
Well-demarcated salmon-colored plaques with silvery micaceous scale; Auspitz sign and Koebner phenomenon.
Diagnosis
Clinical diagnosis; skin biopsy is the gold standard if presentation is atypical.
Treatment
High-potency topical corticosteroids are first-line; avoid systemic steroids due to risk of pustular flare.
Prognosis
Lifelong condition; 30% develop psoriatic arthritis requiring early rheumatology referral.
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Epidemiology & Etiology
Affects 2-3% of the population with a strong genetic component involving the HLA-Cw6 allele. Triggers include streptococcal pharyngitis (guttate type), trauma, and medications like beta-blockers or lithium.
Pertinent Anatomy
Involves the epidermis with hyperproliferation of keratinocytes. The rapid turnover (3-5 days vs 28 days) prevents normal maturation, leading to the characteristic thick, scaly plaques.
Pathophysiology
T-cell mediated autoimmune process involving Th1 and Th17 cells. These cells release cytokines (IL-12, IL-23, TNF-alpha) that drive keratinocyte hyperproliferation and inflammatory infiltration.
Clinical Manifestations
Presents as symmetrical plaques on extensor surfaces (elbows, knees, scalp). Auspitz sign (pinpoint bleeding upon scale removal) and Koebner phenomenon (lesions at sites of trauma) are pathognomonic. Pustular psoriasis can lead to systemic toxicity and requires urgent management.
Diagnosis
Primarily clinical. Skin biopsy is the gold standard, showing parakeratosis (nuclei in stratum corneum) and acanthosis (epidermal hyperplasia).
Treatment
First-line is high-potency topical corticosteroids and vitamin D analogs (calcipotriene). Systemic corticosteroids are strictly contraindicated as they trigger rebound pustular psoriasis. Second-line includes phototherapy (UVB) or biologics like adalimumab for severe disease.
Prognosis
Chronic, relapsing course. Psoriatic arthritis is the most significant complication, characterized by 'pencil-in-cup' deformities on X-ray. Patients have increased risk of metabolic syndrome and cardiovascular disease.
Differential Diagnosis
Seborrheic dermatitis: greasy yellow scales in nasolabial folds
Tinea corporis: central clearing with active, scaly borders
Lichen planus: purple, polygonal, pruritic papules
Pityriasis rosea: herald patch followed by Christmas-tree distribution
Atopic dermatitis: flexural involvement and intense pruritus