Nummular dermatitis

Atopic dermatitis is a chronic, pruritic, relapsing inflammatory skin disease of the face and flexural surfaces driven by a compromised skin barrier and an exaggerated immune response, often accompanying allergic rhinitis and asthma as part of the atopic triad. First-line treatment is fragrance-free emollients plus a topical glucocorticoid for active flares, escalating to topical calcineurin inhibitors, phototherapy, or systemic agents such as dupilumab for disease recalcitrant to topical therapy.

Psoriasis is a chronic, relapsing, immune-mediated dermatosis of accelerated keratinocyte turnover, classically presenting as sharply demarcated, silvery-scaled plaques on extensor surfaces, with guttate, inverse, pustular, and erythrodermic variants and an associated psoriatic arthritis. Mild-to-moderate disease is treated with emollients and topical corticosteroids with or without a vitamin-D analogue (calcipotriene 0.005% twice daily), while widespread (over 10% body surface area), recalcitrant, or severe disease escalates to phototherapy, methotrexate (weekly single dose 7.5 to 25 mg), or a biologic such as adalimumab, ustekinumab, or secukinumab, never a systemic glucocorticoid.

Scabies is an intensely pruritic skin infestation with the mite Sarcoptes scabiei, transmitted by close skin-to-skin contact and classically involving the finger web spaces, wrists, axillae, and genitals while sparing the face and scalp in adults. Diagnosis is clinical, confirmed by mineral oil microscopy when needed, and treatment is topical permethrin or oral ivermectin with simultaneous treatment of contacts.

Pityriasis rosea is an acute, self-limited papulosquamous eruption, often linked to human herpesvirus 6 or 7 reactivation, that begins with a solitary herald patch and is followed by smaller collarette-scaled plaques along skin cleavage lines in a Christmas tree pattern. No treatment is required because the eruption self-resolves, usually within about 6 weeks; oral cetirizine 10 mg once daily can control itching, and topical glucocorticoids relieve symptoms while the rash clears on its own.

Cutaneous T-cell lymphoma is a non-Hodgkin lymphoma of malignant CD4+ T cells that homes to the skin, most often as mycosis fungoides progressing through patch, plaque, and tumor stages, with Sezary syndrome as its aggressive leukemic, erythrodermic variant. Diagnosis rests on skin biopsy showing epidermotropic atypical lymphocytes with Pautrier microabscesses plus clonal T-cell receptor gene rearrangement, and management is staged, from skin-directed therapy in limited disease to systemic agents and extracorporeal photopheresis in advanced or leukemic disease.

Basal cell carcinoma is the most common skin cancer, arising from the basal layer of the epidermis after ultraviolet-driven mutation of the PTCH tumor-suppressor gene, and it rarely metastasizes but can cause significant local tissue destruction if untreated. Diagnosis is confirmed by shave or punch biopsy, and treatment is chosen by risk: standard excision or Mohs micrographic surgery for higher-risk tumors, with curettage and electrodesiccation or topical imiquimod or fluorouracil reserved for low-risk superficial or nodular disease.

Dermatophyte tinea infections are superficial fungal infections of keratinized skin, hair, and nails caused by Trichophyton, Microsporum, and Epidermophyton species, producing an annular scaly plaque with an active advancing border at sites named for their location (corporis, pedis, cruris, capitis, unguium). Diagnosis is clinical, confirmed with a potassium hydroxide preparation showing hyphae, and first-line treatment is a topical azole or allylamine for limited glabrous skin, escalating to oral terbinafine or griseofulvin for hair-bearing or nail disease.

Impetigo is a superficial epidermal infection caused by Staphylococcus aureus or group A streptococcus, presenting as bullous or, more commonly, nonbullous honey-crusted erosions, diagnosed clinically, and treated with topical mupirocin for localized disease or oral antistaphylococcal antibiotics for widespread, bullous, or MRSA-associated infection.