What disorder is characterized by deficits in the mineralization of newly formed bone matrix in the mature skeleton?

The disorder characterized by deficits in the mineralization of newly formed bone matrix in the mature skeleton is osteomalacia. This condition occurs when the bones do not mineralize properly, leading to weakened bones that can become painful and more susceptible to fractures. Osteomalacia is primarily due to a deficiency in vitamin D, calcium, or phosphate, which are essential for proper bone mineralization.

In contrast, osteoporosis is a condition marked by decreased bone density and increased fragility, primarily due to the loss of bone mass rather than mineralization deficits. Paget's disease involves the abnormal remodeling of bone, which can lead to enlarged and misshapen bones, but it does not primarily involve mineralization defects. Sarcoma refers to a type of cancer that originates in connective tissues, including bone, but it is not a disorder related to bone mineralization deficiencies. Therefore, osteomalacia is the correct choice, as it specifically relates to inadequate mineralization in the adult skeleton.