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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Classic psoriasis = well-demarcated erythematous plaques with thick silvery scale on extensor elbows/knees, scalp, lumbosacral area
- Nail pitting, onycholysis, and dactylitis point toward psoriatic arthritis
- Auspitz sign = pinpoint bleeding after scale removal; Koebner phenomenon = plaques at trauma sites
- Mild disease: topical corticosteroids + vitamin D analogs; moderate-severe disease: phototherapy, methotrexate, cyclosporine, apremilast, or biologics
- Screen for psoriatic arthritis, depression, obesity, diabetes, hypertension, dyslipidemia, and cardiovascular risk
Overview
Psoriasis is a chronic inflammatory disease driven by immune activation, especially IL-23/Th17 pathways, causing accelerated keratinocyte proliferation. Plaque psoriasis is the most common form. Other patterns include guttate psoriasis after streptococcal infection, inverse psoriasis in intertriginous areas, pustular psoriasis, erythrodermic psoriasis, and nail psoriasis. Psoriasis is systemic: it is associated with psoriatic arthritis, inflammatory bowel disease, uveitis, nonalcoholic fatty liver disease, metabolic syndrome, and cardiovascular disease.
Epidemiology
Psoriasis affects about 2-3% of adults in the United States. It can begin at any age, with peaks in young adulthood and later middle age. Family history is common, especially with HLA-Cw6. Triggers include infections (especially streptococcal pharyngitis for guttate psoriasis), skin trauma, stress, smoking, alcohol, obesity, HIV, and medications such as lithium, beta-blockers, antimalarials, interferon, and withdrawal of systemic corticosteroids.
Clinical Features
Symptoms
Itchy or painful plaques with recurrent flares and remissions
Joint pain, morning stiffness, swollen digits, heel pain, or back pain suggesting psoriatic arthritis
Recent sore throat followed by numerous small drop-like plaques (guttate psoriasis)
Widespread erythema, fever, chills, malaise, or skin pain in erythrodermic/pustular psoriasis
Signs
Well-demarcated erythematous plaques with thick silvery scale on extensor surfaces
Scalp plaques extending beyond hairline; lumbosacral and umbilical involvement
Auspitz sign: pinpoint bleeding after scale removal
Nail pitting, oil-drop discoloration, onycholysis, subungual hyperkeratosis
Dactylitis, enthesitis, asymmetric oligoarthritis, or DIP arthritis
Investigations
First-line
Clinical diagnosisTypical morphology and distribution are usually sufficient. Assess BSA involvement, special sites, symptoms, and quality-of-life impact
Psoriatic arthritis screenAsk about morning stiffness, swollen joints, dactylitis, enthesitis, inflammatory back pain; early rheumatology referral prevents joint damage
KOH prep if uncertainRule out tinea corporis/cruris when annular or intertriginous plaques mimic psoriasis
Second-line
Skin biopsyRarely needed; shows acanthosis, parakeratosis, neutrophils in stratum corneum (Munro microabscesses), and elongated rete ridges
Baseline labs before systemic therapyCBC, CMP/LFTs, hepatitis B/C, HIV when indicated, TB testing before biologics, pregnancy test when teratogens are considered
Metabolic screeningBP, BMI, HbA1c, lipid panel, smoking/alcohol assessment because cardiometabolic disease is common
Specialist
Rheumatology evaluationFor suspected psoriatic arthritis; imaging may show erosions, new bone formation, pencil-in-cup deformity in advanced disease
Management
AAD-NPF Psoriasis Guidelines1
Mild plaque psoriasis
- Topical corticosteroids are first-line; choose potency by site and duration
- Vitamin D analogs such as calcipotriene are steroid-sparing and often combined with topical steroids
- Topical retinoid (tazarotene), calcineurin inhibitors for face/intertriginous areas, keratolytics for thick scale
- Avoid high-potency steroid on face, groin, axillae, or thin skin because of atrophy risk
2
Moderate-severe or special-site disease
- Narrowband UVB phototherapy for widespread plaque psoriasis when topical therapy is insufficient
- Methotrexate for skin and joint disease; monitor CBC/LFTs and avoid in pregnancy or significant liver disease
- Cyclosporine for rapid control of severe flares; monitor renal function and hypertension
- Apremilast is an oral PDE-4 inhibitor option; monitor weight and mood
- Biologics target TNF-alpha, IL-17, IL-23, or IL-12/23; screen for TB and infections before initiation
3
Acute severe variants
- Erythrodermic or generalized pustular psoriasis: hospitalize if unstable, evaluate infection/medication triggers, urgent dermatology
- Avoid abrupt systemic corticosteroid withdrawal because it can precipitate severe rebound pustular/erythrodermic psoriasis
4
Comorbidity care
- Screen and manage psoriatic arthritis early
- Address obesity, smoking, alcohol excess, diabetes, hypertension, dyslipidemia, depression
- Vaccination review before immunosuppressive therapy; avoid live vaccines during many biologic treatments
Complications
- Psoriatic arthritis: Can cause irreversible erosive joint disease if missed
- Erythrodermic psoriasis: Widespread erythema with thermoregulatory failure, fluid loss, infection risk
- Cardiometabolic disease: Increased risk of obesity, diabetes, hypertension, dyslipidemia, MI, and stroke
- Psychiatric morbidity: Depression, anxiety, stigma, and reduced quality of life
- Treatment toxicity: Methotrexate hepatotoxicity/myelosuppression; cyclosporine nephrotoxicity/HTN; biologic infection risk
USMLE Step 2 CK Exam Tips
- 1Extensor plaques with silvery scale = psoriasis; flexural pruritic eczema = atopic dermatitis
- 2Auspitz sign and Koebner phenomenon are classic psoriasis clues
- 3Nail pitting + DIP arthritis/dactylitis = psoriatic arthritis
- 4Guttate psoriasis after strep throat: sudden drop-like papules in child/young adult
- 5Do not give systemic steroids casually for psoriasis; withdrawal can trigger pustular/erythrodermic flare
- 6Methotrexate is useful for psoriasis with arthritis but is teratogenic and hepatotoxic
- 7Before biologics: screen for TB and hepatitis; avoid live vaccines while immunosuppressed
- 8Inverse psoriasis in skin folds lacks heavy scale and can be mistaken for candidiasis
practicetest your knowledge on psoriasisApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — dermatology and beyond.
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