Why has this 22 year old man developed circular, red lesions on his hands?

The patient A 22-year-old man presented to show me a rash he had just developed. ‘I'm never usually ill,' he said. ‘But I've just got over a really bad attack of cold sores, now this.' He showed me his hands, on which were a number of well defined, red, circular lesions in a symmetrical pattern. ‘I don't know if these are more of the same, doc,' he added, revealing his feet – on which were a few small blisters and less discrete erythematous macules and papules. ‘But they've all come on over the last few days.' Apart from the fact that these lesions were, as he put it, ‘burning and itching', he was otherwise well, with no associated symptoms and no relevant past medical history – and he hadn't been taking any prescribed or OTC medication. First instinct A cursory examination showed that he seemed perfectly well in himself and there were no additional skin lesions to add to the ones he'd already showed me. The acute onset and non-specific nature of the rash immediately made me think of a viral illness – although the lack of accompanying symptoms and the odd appearance of the rash on the hands rendered this a less than cast-iron diagnosis. Differential diagnosis • Viral illness • Drug reaction • Urticaria • Granuloma annulare • Erythema multiforme. In a case like this, the other possibility that normally springs to mind after the default viral pathology is a drug reaction – the flaw in this hypothesis, on this occasion, being his absolute denial that he had taken any medication whatsoever. Urticaria is another rash that can present acutely, and his did itch, but the appearance and pattern certainly weren't typical. The annular lesions on his hands certainly looked like granuloma annulare but the acute presentation made this diagnosis highly unlikely. Which left erythema multiforme as a possibility – albeit a remote one, given the relative rarity of this diagnosis in primary care. The hidden clue As is so often the case, the penny dropped only when I took time to go back over the history more carefully – or, at least, to be more attentive. Because, of course, there had been that preceding episode of herpes simplex – well recognised as a cause of erythema multiforme. Getting on the right track Now that erythema multiforme was firmly on my radar, it was easy to persuade myself that the rash on his hands was a classic example of target lesions. And the rest of the rash was both red and variable – hence, erythema multiforme. Treatment was expectant and within a few weeks his skin had completely cleared up. Dr Keith Hopcroft is a GP in Laindon, Essex Erythema multiforme Erythema multiforme