Differential Diagnosis

Common Diagnoses

  • Acne
  • Scabies
  • Viral Wart and Molluscum Contagiosum
  • Campbell De Morgan Spot
  • Skin Tag

Occasional Diagnoses

Rare Diagnoses

  • Malignant Melanoma, Early Basal Cell Carcinoma, Kaposi’s Sarcoma
  • Darier’s Disease
  • Acanthosis Nigricans
  • Pseudoxanthoma Elasticum
  • Tuberous Sclerosis

Ready Reckoner

Key distinguishing features of the most common diagnoses

AcneScabiesViral WartC. de M. SpotSkin Tag
Characteristic DistributionYesYesNoYesNo
Associated LesionsYesYesNoNoNo
History of ContactNoYesPossibleNoNo

Possible Investigations

  • In practice, very few investigations are needed with this presentation: A lipid screen is required in the presence of xanthomata; genital warts require referral for screening for other STDs; thorough investigation may be needed in the very rare case where underlying malignancy is possible (e.g. acanthosis nigricans); and obscure rashes or solitary papules may occasionally require excision biopsy for a definitive diagnosis.

Top Tips

  • Bear in mind that skin cancer is usually uppermost in the patient’s mind, especially in subacute or chronic cases – so provide appropriate reassurance.
  • In obscure solitary lesions, record clinical findings carefully and arrange to review in due course.
  • Itchy, asymmetrical grouped papules are likely to be insect bites, although the patient may take some convincing!

Red Flags

  • An enlarging dark blue or blue-black papule may be a malignant melanoma, blue naevus or Kaposi’s sarcoma. Refer for urgent opinion.
  • Brown, skin-coloured papules crowded around the nose of a child may be tuberous sclerosis. This can be associated with serious systemic pathology so refer for expert opinion.
  • An intensely itchy papular rash which is worse at night and has no other obvious cause is likely to be scabies, even if scabetic burrows are not evident – treat on suspicion.

Published: 2nd August 2022 Updated: 10th April 2024

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