Clues Pointing TO This Diagnosis

Thick, scaly, red plaques on the elbows — extensor surfaces such as elbows and knees are the classic sites for plaque psoriasis
Symmetric distribution on elbows, knees, and scalp — plaque psoriasis is characteristically well-demarcated and often symmetric, with the scalp a common site
Well-demarcated ("sharply demarcated") plaques — sharp demarcation is a hallmark distinguishing psoriasis from eczema
Salmon/red color with silvery scale — the salmon-pink plaque with overlying scale is typical in lighter skin types
Positive Auspitz sign (pinpoint bleeding when scale is scraped) — lifting the scale reveals pinpoint bleeding, a classic psoriasis finding
Nail pitting and onycholysis (separation at the tip) — nail pitting and onycholysis are characteristic of psoriatic nail disease
Onset in her twenties with a chronic, lifelong course — psoriasis is a chronic immune-mediated disease that commonly begins in adulthood
Worse with stress — emotional stress is a well-recognized trigger/exacerbator
Winter worsening / summer improvement — a seasonal pattern (worse in winter, better with sun exposure) is typical
Joint aching in fingers and lower back — psoriatic arthritis occurs in a subset of patients and involves peripheral and axial joints
Moderate disease requiring more than topical therapy — extensive involvement plus joint disease escalates management beyond topicals alone
Awareness of triggers (stress, strep, certain drugs) — infection (especially streptococcal), stress, and drugs (beta-blockers, lithium, antimalarials, NSAIDs) are established triggers

Why It's NOT the Similar Diseases

NOT atopic dermatitis (eczema) — eczema lacks the silvery scale and favors flexor surfaces, whereas Mabel's plaques are silvery-scaled and on extensor surfaces (elbows/knees)
NOT tinea (dermatophyte) infection — tinea shows central clearing (annular with active border), unlike Mabel's uniformly thick, well-demarcated plaques with a positive Auspitz sign
NOT seborrheic dermatitis — seborrheic dermatitis produces greasy yellowish scale in sebaceous areas (nasolabial folds, eyebrows), not thick silvery plaques on elbows/knees with nail pitting
NOT squamous cell carcinoma — SCC arises in sun-exposed skin as a solitary lesion, whereas Mabel has symmetric plaques on less sun-exposed extensor surfaces
NOT inverse (intertriginous) psoriasis alone — inverse psoriasis occurs in skin folds and lacks scale, but Mabel's disease is classic scaly plaque psoriasis on extensor surfaces and scalp

Differential Diagnosis List

Atopic dermatitis — ruled out by silvery scale and extensor (not flexor) distribution
Tinea corporis — ruled out by absence of central clearing and presence of the Auspitz sign
Seborrheic dermatitis — ruled out by thick silvery plaques on extensor surfaces rather than greasy scale in sebaceous zones
Squamous cell carcinoma — ruled out by symmetric multifocal plaques on less sun-exposed skin
Nummular eczema — ruled out by well-demarcated plaques with silvery scale, Auspitz sign, and nail pitting

Key Distinguishing Test

Psoriasis is a clinical diagnosis based on characteristic well-demarcated, erythematous, silvery-scaled plaques in typical locations (elbows, knees, scalp, lower back), supported by findings such as the Auspitz sign and nail pitting.
Skin biopsy is rarely needed and is reserved for atypical presentations, showing acanthosis with parakeratosis and neutrophilic microabscesses.

Board Note

Diagnosis is clinical; look for well-demarcated plaques on extensor surfaces/scalp, nail pitting/onycholysis, and the Auspitz sign, and screen every patient for psoriatic arthritis, which occurs in roughly 5–25% and drives more aggressive treatment
First-line for mild/localized disease is topical therapy—topical corticosteroids (potency matched to body site) and vitamin D analogues, with the corticosteroid–vitamin D combination more effective than either alone; use low-potency steroids on face/folds and reserve superpotent agents for thick plaques
Phototherapy (narrowband UVB, typically 3×/week) is an effective option for moderate or more extensive disease
Systemic therapy (biologics or nonbiologics such as methotrexate) is indicated for moderate-to-severe disease or when psoriatic arthritis is present; because psoriatic arthritis is present, choose a therapy that treats both skin and joints
TNF-α inhibitors treat both psoriasis and psoriatic arthritis; screen for latent tuberculosis and hepatitis B before starting, and counsel that class effects include injection-site reactions and upper respiratory/nasopharyngeal infections
Counsel on triggers—streptococcal infection, stress, smoking, obesity, alcohol, and drugs (beta-blockers, lithium, antimalarials, NSAIDs, systemic steroid withdrawal)—and address cardiometabolic and depression comorbidities
Refer to dermatology for moderate-to-severe or refractory disease and to rheumatology for suspected psoriatic arthritis; urgent referral is warranted for erythrodermic or generalized pustular psoriasis
The following treatment algorithm from the JAMA review shows how the presence of psoriatic arthritis and disease severity guide escalation from topicals to phototherapy to systemics—the exact decision points in Mabel's case:
Figure 3. Overall Treatment Approach for Plaque Psoriasis
Pathophysiology, Clinical Presentation, and Treatment of Psoriasis: A Review. JAMA. May 18, 2020.
Content used under license from the JAMA Network®
© American Medical Association

“Mabel’s Story”