How does osteoporosis primarily affect bone integrity?

Osteoporosis is primarily characterized by a reduction in bone density, which leads to increased brittleness. This occurs because the condition involves a decrease in the amount of bone mass and strength, making bones more susceptible to fractures and breaks under stress. The overall integrity and structural quality of the bone deteriorate, and as a result, even minor falls or bumps can lead to serious injuries.

In contrast, the other options suggest effects that do not align with the nature of osteoporosis. Strengthening or increasing flexibility in bones would imply an improvement in bone density and strength, which is contrary to what occurs in osteoporosis. Similarly, while irregular shaping of bones might occur in various other conditions, the defining feature of osteoporosis is the weakening of bone, not its shape alone. Thus, the primary impact of osteoporosis is the increased brittleness, making option C the accurate choice.