Which one of the following is a recognized breast cancer risk factor?

Starting menarche before the age of 12 is a recognized breast cancer risk factor due to its association with prolonged exposure to estrogen. Early menarche can lead to a longer lifetime of menstrual cycles, resulting in more cumulative estrogen exposure. Estrogen is known to play a significant role in breast tissue development and potentially in the pathogenesis of breast cancer. This increased exposure can elevate the risk of developing breast cancer later in life. On the other hand, starting menarche after age 12 generally correlates with a reduced exposure duration and is therefore considered protective against breast cancer.

Having a first child before age 20 is typically regarded as a protective factor against breast cancer, as pregnancy induces changes in breast tissue that can reduce the risk. Engaging in regular physical exercise is also associated with a reduced risk of breast cancer, as it helps to maintain a healthy weight and balance hormone levels.