At what stage does the areola commonly darken and what is the reason for this?

The correct answer identifies that the areola commonly darkens during puberty due to increased melanin production. During puberty, the body undergoes significant hormonal changes that lead to the stimulation of melanocytes, the cells responsible for producing melanin, the pigment that gives color to the skin and other tissues. This increase in melanin can result in the darkening of the areola as the body prepares for potential future functions related to lactation and sexual maturity.

The other stages mentioned, such as pregnancy, menopause, and infancy, do involve changes in the areola's appearance, but they do not primarily relate to the mechanism of increased melanin production as seen during puberty. During pregnancy, hormonal fluctuations do cause some darkening due to an increase in several hormones, but this is not the initial stage. After menopause, the changes are often related to skin aging rather than pigmentation changes. In infancy, any changes observed are not due to melanin production but rather are associated with exposure to breast milk, which does not impact pigmentation in the way puberty does.