What bone disease is characterized by decreased bone mass and an increased risk of fractures?

Osteoporosis is the correct choice because it specifically refers to a condition where there is a reduction in bone density, leading to weaker bones that are more susceptible to fractures. This disease typically occurs as a result of an imbalance between bone resorption and bone formation, leading to a net loss in bone mass over time.

The condition is often asymptomatic in its early stages but can result in significant health issues when fractures occur, particularly in the hip, spine, and wrist. Osteoporosis is commonly associated with aging, hormonal changes, and lifestyle factors such as diet and physical activity levels.

Other diseases mentioned can affect the bones but in different ways. For instance, osteoarthritis primarily affects the cartilage and joint structures rather than bone density. Rickets is a disease caused by a deficiency in vitamin D, calcium, or phosphate, leading to soft and weakened bones in children, while Paget's disease results in the abnormal breakdown and formation of bone, often leading to enlarged bones and not specifically to decreased bone mass characteristic of osteoporosis.