Scales and Plaques

Differential Diagnosis

Common Diagnoses

Rare Diagnoses

  • Malignancy: Bowen’s Disease and Mycosis Fungoides (Cutaneous T-Cell Lymphoma)
  • Drug Induced (e.g. Β-Blockers and Carbamazepine)
  • Ichthyosis (Various Forms)
  • Keratoderma Blenorrhagica (Part of Reiter’s Syndrome)
  • Pityriasis Lichenoides Chronica
  • Secondary Syphilis

Ready Reckoner

Key distinguishing features of the most common diagnoses

Psoriasis EczemaFungalSeborrhoeic DermatitisSeborrhoeic Keratosis
Well-Demarcated BorderYesNoYesNoYes
Markedly ItchyNoYesPossiblePossibleNo
Severe DandruffPossiblePossiblePossibleYesNo

Possible Investigations


Possible:Wood’s light, skin scrapings/hair samples, patch testing.

Small Print:Skin biopsy, syphilis serology, FBC, ESR/CRP, fasting glucose or HbA1c.

  • Green fluorescence under UV (Wood’s) light is diagnostic of microsporum fungal infection.
  • Skin scrapings and hair samples for mycology: Will help differentiate fungal infections from similar rashes.
  • Skin biopsy: May be the only way to achieve a firm diagnosis in obscure rashes, and is essential if malignancy suspected.
  • Patch testing: To establish the likely allergen in allergic contact eczema
  • Syphilis serology: If justified by clinical features or obscure pattern.
  • FBC and ESR/CRP: May suggest significant underlying disease (e.g. T-cell lymphoma); ESR/CRP also elevated in Reiter’s disease
  • Fasting glucose or HbA1c: Check for diabetes in recurrent fungal infections.

Top Tips

  • To help distinguish between fungal and eczematous rashes, look at the symmetry and edges of the lesions. Fungal rashes are usually asymmetrical with a scaly, raised edge.
  • In the presence of a fungal rash, look for infection elsewhere (e.g. groins and feet) and treat both areas, otherwise the problem is likely to recur.
  • In uncertain cases, explain to the patient that the real diagnosis may only become apparent as the rash develops (the typical example being the herald patch of pityriasis rosea looking like initially tinea corporis) – invite the patient to return for reassessment if your initial treatment proves unsuccessful.
  • A symmetrical, glazed, scaly and fissured rash on the soles of a trainer-loving child or adolescent is juvenile plantar dermatosis.
  • Eight percent of people with psoriasis will have arthropathy, which is usually also associated with nail changes. Check the nails and ask about joint symptoms in patients with psoriasis.

Red Flags

  • Erythroderma – universal redness and scaling caused, rarely, by psoriasis, eczema, mycosis fungoides and drug eruptions – renders the patient systemically unwell. Urgent inpatient treatment is required.
  • A solitary, well-defined, slowly growing scaly plaque on the face, hands or legs of the middleaged or elderly patient is probably Bowen’s disease – but it can easily be mistaken for an isolated patch of eczema or psoriasis.
  • If a pityriasis rosea-like rash extends to the palms and soles, with fever, malaise, sore throat and lymphadenopathy, consider secondary syphilis.
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