Which is pathognomic lesion of acne vulgaris?

Comedones are pathognomic of acne. Comedones are the keratin plugs of the pilosebaceous ducts. When the keratin plug is inside the follicle, it is called closed comedone (whitehead). When the keratin plug is exposed to the environment, it turns black and is known as open comedone (blackhead). Comedonal lesions are the earliest lesions of acne. Typical acne lesions are comedones, inflammatory papules, pustules and nodules.