Final answer:
Acne nodules are caused by the bacterial infection of sebum-clogged pores that leads to inflammation and the accumulation of pus.
Step-by-step explanation:
Nodules are one of the skin lesions that can occur when hair follicles become clogged with sebum, dead skin cells, and sometimes bacteria. This blockage can lead to an environment conducive to the growth of certain bacteria, such as Propionibacterium acnes and Staphylococcus. When bacteria infect these sebum-clogged pores, they can cause an infection, which the immune system responds to with inflammation. This inflammation can then cause swelling and redness, potentially resulting in the formation of a pus-filled lesion known as an acne nodule. Moreover, hormonal changes, especially during puberty, can stimulate the release of sebum, contributing to the development of acne.
Therefore, in the context of acne, the answer to the question 'What causes a nodule?' is the accumulation of pus due to the bacterial infection of a sebum-clogged pore.