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Knee lesions

The parents of a 6-year-old boy were quite concerned about several lesions on the child’s left knee. The asymptomatic blisters had first appeared about 2 weeks prior. A topical steroid cream prescribed by the family’s primary care provider had not helped.

No one else in the family was similarly affected. They all were reportedly quite healthy, although all were atopic—prone to seasonal allergies and eczema.

Examination revealed at least 6 round, scaly, red lesions on the patient’s knee, ranging from 3 mm to 3 cm in diameter. According to the parents, these had first appeared as intact blisters. There was little to no tenderness or redness around the lesions, and there was no palpable adenopathy in the groin.

The child was in no distress and was afebrile.

What is the most likely diagnostic explanation for these lesions?

Bullous impetigo

MRSA-caused cellulitis

Psoriasis vulgaris

Nummular eczema


The correct answer is bullous impetigo (choice “a”).

Had the source of the problem been MRSA (choice “b”), there would have been marked tenderness and swelling. Psoriasis vulgaris (choice “c”) was a possibility; however, it almost never manifests with blisters, it rarely comes on as quickly as this patient’s problem did, and it produces scale that is far thicker and more tenacious than that seen in bullous impetigo. Nummular eczema (choice “d”) does not manifest with blisters and would likely have caused itching.


Impetigo is one of many common skin diseases—other examples include granuloma annulare and lichen planus—that have bullous variants, which can make diagnosis challenging. Impetigo is easy to diagnose in its more common papulosquamous form. But it also can manifest with flaccid blisters that last only a short time, leaving the round, scaly lesions seen in this case.

Staphylococcus aureus, the organism responsible for bullous impetigo, elaborates serine proteases, which bind to and cleave desmoglein-1. This effectively destroys the connections between skin layers, creating a space that is quickly filled with serum. The level of this separation is typically subcorneal, which allows the formation of a very thin, friable roof for the bulla.

This modern version of impetigo is not considered dangerous, despite its association with staph aureus. In pre-antibiotic times, the predominant organism was streptococcal, some types of which could be nephritogenic—that is, capable of causing Bright disease or, as it is now known, acute post-streptococcal glomerulonephritis. Fortunately, this potentially fatal condition is only rarely seen in modern times.

Two items of note about this case: Bullous impetigo, contrary to what was seen in this patient, typically favors intertriginous areas. And a key factor is the history of atopy, which renders the patient more susceptible to skin infections of all kinds.


The patient responded well to topical mupirocin ointment, applied three times a day. In rare instances, impetigo can require an oral antibiotic, such as cephalexin.

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