PassFinals
Dermatology

Fungal skin infections

Dermatophyte fungi digest keratin in the stratum corneum and spread outward as they consume it, producing a scaly lesion with an actively advancing, inflamed edge and a clearing centre, the ring that gives ringworm its name.

In a nutshell

Dermatophytes digest keratin and spread outward from the point of inoculation, so the advancing edge shows the most fungus and inflammation while the centre, already consumed, heals and clears, producing the classic ring. Site determines the clinical name and appearance.

Classic presentation

An itchy, scaly annular lesion with a well-defined active edge and central clearing, or fungal changes at the feet, groin, scalp or nails depending on site.

Key points

  • Dermatophytes only infect dead keratin, which is why infection stays superficial in immunocompetent hosts.
  • The active edge, not the centre, is where fungal density is highest: that's where scrapings should be taken.
  • Tinea capitis invades the hair shaft, causing broken hairs and alopecia, and can progress to a kerion risking scarring.
  • Onychomycosis and scalp infection need oral antifungals because topical treatment cannot penetrate nail or hair keratin adequately.
  • Topical corticosteroids used alone on an unrecognised fungal infection cause tinea incognito, an atypical, worsened, harder-to-diagnose infection.

First-line investigation

Skin, hair or nail scrapings from the active edge for microscopy and culture to confirm the dermatophyte diagnosis.

Management

Recognise the site and severity

  • Confirm the pattern and involved site; urgent review is needed for tinea capitis, kerion, scarring alopecia, extensive disease, secondary infection or a high-risk host.2,4

Confirm when treatment is consequential

  • Use skin, nail or hair mycology when the appearance is atypical, treatment has failed, scalp or nails are involved, or oral therapy is being considered; nail infection should be confirmed before systemic treatment.1,6,2

Treat localised skin infection

  • Use an appropriate topical antifungal for localised skin, foot or groin infection according to current NICE CKS and BNF advice, and keep affected skin dry.1,5

Use systemic treatment when topical therapy cannot reach

  • Tinea capitis requires oral treatment plus an adjunctive antifungal shampoo; confirmed clinically significant onychomycosis may need oral therapy after interaction and liver-risk review.2,6,3,5

Prevent spread and reassess failure

  • Avoid sharing towels, footwear, combs or bedding, check relevant contacts and pets, and reassess the diagnosis or refer if infection fails to improve or recurs.4,2,3

Exam traps

  • A ring-shaped rash that has been treated with a topical steroid and got worse or changed shape suggests tinea incognito, not a resistant eczema.
  • Tinea capitis needs an oral antifungal: topical treatment alone will not clear infection within the hair shaft.
  • Interdigital foot maceration is often fungal (tinea pedis) even without a classic ring, especially with associated scaling elsewhere on the sole.

Illustrations

Tinea corporis ring lesionClinical photograph of an annular scaly plaque with an active raised edge and central clearing on the trunk.డా. గన్నవరపు నరసింహమూర్తి, Wikimedia Commons · CC0
Dermatophyte keratin invasionDiagram showing a dermatophyte digesting keratin at the advancing edge of a lesion while the centre behind it heals and clears.PassFinals · original
OnychomycosisClinical photograph of a thickened, discoloured, crumbling toenail typical of fungal nail infection.James Heilman, MD, Wikimedia Commons · CC-BY-SA-3.0

Key sources

  1. NICE CKS: Fungal skin infection — body and groin (Current UK primary-care guidance for recognition, differential diagnosis and topical/systemic management of common dermatophyte infection of skin, feet and groin; professional access may require NHS or institutional login.)
  2. British Association of Dermatologists: Tinea capitis (May 2023) (UK dermatology patient information covering scalp diagnosis, mycology, oral treatment plus adjunctive shampoo, kerion and scarring-alopecia risk, contact and pet control.)
  3. British Association of Dermatologists: Fungal nail infections (updated April 2026) (Current UK dermatology information on confirming nail infection, topical and oral treatment options, nail care, drug safety and expected slow nail recovery.)
  4. NHS: Ringworm (NHS information on typical presentations, pharmacy treatment, scalp referral, immunosuppression, transmission and hygiene; last reviewed 3 August 2023 with the page review date noted by NHS.)
  5. BNF online: current antifungal prescribing (Use the current BNF and local dermatology or microbiology protocol for antifungal choice, formulation, dose, duration, age restrictions, interactions, hepatic monitoring and pregnancy advice; no fixed regimen is reproduced here.)
  6. NICE CKS: Fungal nail infection (Current UK primary-care guidance for confirming suspected fungal nail infection, treatment selection and systemic antifungal safety; professional access may require NHS or institutional login.)

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.