A woman with dense fibrous and glandular tissue in her breast on a baseline mammogram is typically:

The presence of dense fibrous and glandular tissue on a mammogram is more common in younger women, particularly those who are younger than 45 years. This is primarily due to hormonal influences that dictate breast tissue composition. As women age, particularly after menopause, the amount of glandular and fibrous tissue typically decreases, and fatty tissue becomes more prevalent, leading to less dense mammograms.

Dense breast tissue can make it more challenging to detect abnormalities, which is why understanding the age-related changes in breast composition is crucial for mammographers. In younger women, the presence of dense tissue is a normal physiological characteristic, aligning with the correct answer regarding age.