Explanation: Acne vulgaris is a chronic inflammatory skin condition that affects the hair follicles and sebaceous glands. It is characterized by the formation of comedones (blackheads and whiteheads), papules, pustules, nodules, and cysts. While it is not caused by a viral, bacterial, or fungal infection, it is associated with factors such as hormonal imbalances, increased sebum production, and inflammation of the hair follicles.

Incorrect choices: a. A viral skin infection - This is incorrect because acne vulgaris is not caused by a viral infection. It is primarily a result of the abnormal keratinization of hair follicles and excessive sebum production.

b. A bacterial skin infection - This is incorrect because although bacteria can play a role in the development of acne, it is not the primary cause. Propionibacterium acnes, a type of bacteria, is commonly found on the skin and can contribute to inflammation in acne lesions, but it is not the initial cause of acne.

d. A fungal skin infection - This is incorrect because acne vulgaris is not caused by a fungal infection. It is not related to fungi or yeast but is rather a disorder of the pilosebaceous unit in the skin.