**Core Concept**
Osteoporosis is a chronic bone disease characterized by a decrease in bone mass and density, leading to an increased risk of fractures. It involves an imbalance between bone resorption and bone formation, with the latter being reduced.

**Why the Correct Answer is Right**
The correct answer involves identifying the correct cause of osteoporosis. Osteoporosis is primarily caused by a combination of genetic, hormonal, and lifestyle factors. Hormonal changes, such as those seen in postmenopausal women due to decreased estrogen levels, can lead to an increase in bone resorption. Additionally, a lack of vitamin D and calcium can contribute to osteoporosis. Furthermore, age-related decline in physical activity and muscle mass also play a crucial role.

**Why Each Wrong Option is Incorrect**
**Option A:** While age is a significant risk factor for osteoporosis, it is not a direct cause of the disease.
**Option B:** Sedentary lifestyle can contribute to osteoporosis, but it is not a primary cause.
**Option C:** Smoking is a risk factor for osteoporosis, but it is not a direct cause of the disease.

**Clinical Pearl / High-Yield Fact**
It is essential to note that osteoporosis can be prevented or delayed with a combination of regular exercise, a balanced diet rich in calcium and vitamin D, and adequate hormone replacement therapy in postmenopausal women.

**Correct Answer:** D. Smoking is a risk factor for osteoporosis, but it is not a direct cause of the disease.