<prompt> Erythema multiforme is most commonly caused by?Herpes simplexIdiopathicTBDrugs  <response>: Ans. is b i.e., Idiopathic Idiopathic erythema multiforme is the most common cause of EM. Herpes simplex is the most impoant infectious cause of EM. Causes of Erythema multiforme Idiopathic -- Most common cause Viral -- HSV most impoant HBV, Mumps, Adenovirus Bacteria - Streptococci, tuberculosis Fungal -- Coccidioidomycosis, Histoplasmosis. Drugs --- Antibiotics Sulphonamide, Phenytoin, NSAIDS. Autoimmune disease -- SLE, thyroiditis, RA Others -- Sarcoidosis, Pregnancy, Malignancy. 