Acne spot. Close up of a spot on human skin caused by acne vulgaris. The yellowish centre is surrounded by a red, inflamed area. Acne vulgaris is a skin disorder caused by obstruction of hair follicles (the pits in which hairs grow) by a plug of sebum, an oily skin-lubricant secreted by seba- ceous glands. Bacterial infection of the blocked follicle causes build up of yellow pus inside the spot. Acne vulgaris is most common in adolescents, where it is thought to be linked to the hormonal changes that occur with puberty. The condition usually disappears in early adulthood. Severe cases may require antibiotic or retinoid drugs to prevent permanent scarring.