Which of the following is a risk factor for nonunion of a fracture?

Smoking is a well-documented risk factor for nonunion of a fracture due to its adverse effects on bone healing. The chemicals present in cigarettes reduce blood flow to the bones and impair the function of osteoblasts, which are essential for new bone formation. Additionally, smoking can interfere with the inflammatory responses needed for the healing process, thus prolonging healing time and increasing the likelihood of complications such as nonunion.

In contrast, while a low calcium diet can contribute to poor bone health, it does not directly impede the healing process to the same extent as smoking. A high activity level might actually promote healing in some cases, depending on the nature of the activity and stage of healing. Young age is typically associated with better healing capabilities due to stronger regenerative potential and metabolic activity.