This is a 15-year-old female patient, with no family or personal history of interest, who comes to the clinic for the insidious appearance, 3-4 days previously, of erythematous lesions on the pretibial surface of both legs, symmetrical in shape, which have increased in number and size and some have begun to change colour, with a certain yellowish colour.

The patient reported a mild upper airway infection the previous week with no other symptoms. She denies taking any medication.
Physical examination revealed no fever. Blood pressure 110/70 mm Hg, heart rate: 60 beats per minute and normal cardiorespiratory auscultation.
The lesions appeared as erythematous skin nodules, slightly hyperthermic and painful on palpation, ranging in diameter from 1 to 3 cm on the pretibial surface, with no lesions observed at other sites.
The patient was recommended rest and analgesia with paracetamol or NSAIDs, and was referred to the dermatology department for a confirmatory biopsy.
Given that the patient's physical examination was normal, the following complementary tests were requested to reach an aetiological diagnosis: haematology, formula, blood count and ESR; biochemistry with liver, kidney and thyroid function tests; proteinogram; calcium and phosphorus; rheumatoid factor, C-reactive protein (CRP) and antinuclear antibodies (ANA). All values were within the normal range. A chest X-ray was also requested, with no pathological findings; Mantoux was negative; ASLO was negative and Epstein Barr virus (EBV) serology was positive for IgG, negative for IgM; pregnancy test was negative.
After 15 days the lesions had practically disappeared. The biopsy gave the definitive diagnosis. Given the results of the complementary tests and the patient's evolution, it was labelled as erythema nodosum of idiopathic aetiology.