Rashes and Skin Lesions in Primary Care: Common Patterns, Topical Steroid Potency and When to Biopsy or Refer

Skin questions on the NP boards reward a disciplined sequence: describe the lesion, name the pattern, confirm the cheap test when the pattern is ambiguous, then pick a treatment matched to the site and severity. Name a rash by its morphology and distribution, confirm a suspected fungal infection with a potassium hydroxide (KOH) preparation before using a steroid, and match topical steroid potency to the body site: low potency for the face, skin folds and infants, higher potency for thick plaques and the palms and soles. Any pigmented lesion that meets ABCDE criteria or stands out as an ugly duckling, and any nonhealing, bleeding or enlarging lesion, needs a biopsy or dermatology referral.

Step 1: describe before diagnosing

Exam stems describe lesions in dermatologic language, and the description usually gives the diagnosis away. Know the primary terms: macule (flat, under 1 cm) and patch (flat, over 1 cm); papule (raised, under 1 cm) and plaque (raised, over 1 cm); vesicle and bulla (fluid-filled, under and over 1 cm); pustule; nodule (deeper, firm). Add the secondary features: scale, crust, lichenification, excoriation, erosion or ulceration. Then distribution: flexural, extensor, sun-exposed, dermatomal, intertriginous, or following a line of contact.

Step 2: recognize the common patterns

Atopic dermatitis under the 2023 AAD topical guideline

The American Academy of Dermatology (AAD) strongly recommends moisturizers, topical corticosteroids, topical calcineurin inhibitors (tacrolimus, pimecrolimus), a topical phosphodiesterase-4 inhibitor (crisaborole) and a topical JAK inhibitor (ruxolitinib) for adults with atopic dermatitis. Bathing practices and wet wrap therapy receive conditional recommendations, and topical antimicrobials, antiseptics and topical antihistamines are conditionally recommended against. Practical points:

• Moisturize at least daily, ideally after bathing, as both treatment and prevention of flares.
• Treat flares with a steroid matched to site and severity; use a calcineurin inhibitor for the face, eyelids and folds where steroid atrophy is a concern.
• Intermittent maintenance application to flare-prone sites (for example twice weekly) reduces relapse.
• The guideline flags topical steroid withdrawal, linked mainly to prolonged, inappropriate use of potent steroids on the face and folds.
• Moderate to severe disease not controlled with topical therapy is a dermatology referral for phototherapy, dupilumab or other systemic agents. Oral glucocorticoids are not a maintenance treatment.

Step 3: choose the topical steroid potency

Topical corticosteroids are ranked from class 1 (superpotent) to class 7 (least potent). Vehicle matters too: ointments are more potent and less irritating than creams of the same drug, and solutions, foams and gels suit hairy areas.

Rules the exam tests:

• Thin skin (face, eyelids, genitals, intertriginous folds) absorbs far more drug than palms and soles, and occlusion increases absorption further, so folds behave like occluded skin.
• Infants and older adults have a higher surface-area-to-weight ratio or thinner skin and are at higher risk of systemic absorption.
• Local adverse effects: atrophy, striae, telangiectasia, hypopigmentation, perioral dermatitis, acneiform eruptions, and masking or worsening of tinea (tinea incognito).
• Systemic adverse effects with potent agents over large areas or long periods: hypothalamic-pituitary-adrenal axis suppression, hyperglycemia.
• Quantity: one fingertip unit (about 0.5 g) covers an area the size of two adult palms.
• Never use a combination steroid and antifungal cream as a substitute for diagnosis; treat confirmed tinea with an antifungal alone.

Acne under the 2024 AAD guideline

The AAD strongly recommends benzoyl peroxide, topical retinoids, topical antibiotics, oral doxycycline and, for the right patient, oral isotretinoin. It conditionally recommends topical clascoterone, azelaic acid and salicylic acid, oral minocycline and sarecycline, combined oral contraceptives and spironolactone.

Doxycycline causes photosensitivity and esophagitis (take with water, stay upright), and tetracyclines are avoided in pregnancy and in children under 8. Isotretinoin is a referral decision for most primary care providers.

Step 4: evaluate pigmented and suspicious lesions

ABCDE and the ugly duckling

The ABCDE criteria for melanoma (expanded with E for evolving by Abbasi and colleagues in 2004):

• A, asymmetry: one half does not match the other.
• B, border irregularity: ragged, notched or blurred edges.
• C, color variegation: several shades of brown, black, red, white or blue.
• D, diameter over 6 mm, though melanomas can be smaller.
• E, evolving: any change in size, shape, color, elevation, or new symptoms such as bleeding, itching or crusting. Evolution is the most important single feature.

The ugly duckling sign (Grob and Bonerandi, 1998) compares a lesion with the patient's other moles. Most nevi in one person look alike; the one that looks different from the rest is suspicious even if it does not meet ABCDE criteria. This is how nodular and amelanotic melanomas, which often fail ABCDE, are caught.

Nonpigmented skin cancers and precursors

Step 5: choose the biopsy, or refer

The 2019 AAD melanoma guideline prefers a narrow excisional biopsy with 1 to 3 mm margins for a lesion suspicious for melanoma, because the pathologist needs the full Breslow depth to stage it. A partial biopsy (deep saucerization shave or punch) is acceptable for a large lesion or a cosmetically sensitive site, but a superficial shave of a suspected melanoma risks transecting the base and understaging the tumor.

Refer to dermatology when:

• A pigmented lesion meets ABCDE criteria or is an ugly duckling and the provider does not perform excisional biopsies.
• A biopsy shows melanoma, or squamous cell carcinoma with high-risk features.
• A suspected skin cancer is on the face, ears, lips, eyelids, genitals, hands or feet.
• The patient is immunosuppressed, especially an organ transplant recipient.
• A rash fails to respond after an adequate trial of first-line therapy or the diagnosis is uncertain.
• Moderate to severe psoriasis or atopic dermatitis may need phototherapy or systemic therapy, or severe acne needs isotretinoin.

Same-day or emergency care: a widespread blistering rash with mucosal involvement (Stevens-Johnson syndrome or toxic epidermal necrolysis, often after a new drug such as allopurinol, lamotrigine or a sulfonamide), a rapidly spreading cellulitis with systemic signs, or a petechial rash with fever.

Screening and follow-up

The 2023 US Preventive Services Task Force (USPSTF) statement gives an I grade: evidence is insufficient to assess the benefits and harms of a clinician visual skin examination to screen asymptomatic adolescents and adults with no history of premalignant or malignant skin lesions. That statement does not apply to a patient who reports or shows a changing or suspicious lesion; those patients need evaluation. Patients with a prior melanoma, many atypical nevi, a strong family history or organ transplantation are followed by dermatology on a schedule the specialist sets.

Follow-up for inflammatory rashes: reassess atopic dermatitis and psoriasis in 2 to 4 weeks after starting a potent steroid, step down potency once controlled, and check acne response at 8 to 12 weeks, since topical regimens take that long.

Patient and family education

Teach the soak-and-seal routine for eczema (short lukewarm bath, pat dry, moisturizer within minutes), how much steroid to apply using fingertip units, and how long to use a potent product before stepping down. Explain that topical steroids used correctly are safe, which counters steroid phobia that leads to undertreatment. For acne, set expectations of 2 to 3 months before improvement and warn about initial retinoid irritation and benzoyl peroxide bleaching fabric. For scabies, the whole household is treated at once and bedding is washed hot. For skin cancer, teach monthly self-examination using the ABCDE rule, including scalp, soles and nail beds, and sun protection: broad-spectrum sunscreen, protective clothing and avoidance of tanning beds. Caregivers of older adults can check the back and scalp the patient cannot see.

How the exam tests this

• Steroid for an annular scaly plaque. The answer is a KOH preparation or an antifungal; a steroid causes tinea incognito.
• Superpotent steroid on the face or groin. Wrong site: low potency or a calcineurin inhibitor.
• Bilateral "cellulitis." Bilateral red lower legs with edema and brown discoloration is stasis dermatitis.
• Shave biopsy of a suspected melanoma. Excisional biopsy with 1 to 3 mm margins is preferred.
• A small but changing mole. Evolution outweighs diameter; biopsy or refer.
• Annual skin screening as a USPSTF recommendation. The grade is I for asymptomatic people without prior skin cancer.
• Oral antibiotic monotherapy for acne. Always combine with topical therapy including benzoyl peroxide.
• Pityriasis rosea in a sexually active patient without considering secondary syphilis.

Practice question

A 46-year-old man asks about a mole on his upper back that his wife says has changed over the past 4 months. The lesion is 5 mm, asymmetric, with an irregular border and two shades of brown with a black area. His other nevi are uniform, round and light brown. He has no personal history of skin cancer. Which of the following is the most appropriate next step?

• A. Reassure him because the lesion is smaller than 6 mm and photograph it for comparison in 12 months
• B. Perform a superficial shave biopsy at the level of the epidermis
• C. Perform an excisional biopsy with 1 to 3 mm margins or refer urgently to dermatology for excision
• D. Apply triamcinolone 0.1 percent cream for 2 weeks and reassess

Answer: C. The lesion has asymmetry, border irregularity, color variegation and documented evolution, and it is an ugly duckling compared with his other nevi, so melanoma must be excluded with a narrow excisional biopsy that allows full depth measurement. Option A is wrong because diameter under 6 mm does not exclude melanoma, and change over time is the most worrying feature. Option B risks transecting the lesion and understaging a melanoma, since Breslow depth determines staging and surgical margins. Option D treats a suspicious pigmented lesion as an inflammatory rash and delays diagnosis.

Key points

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