**Core Concept**
Osteoporosis is a chronic metabolic bone disease characterized by a decrease in bone mass and density, leading to an increased risk of fractures. This condition results from an imbalance between bone resorption and bone formation, often due to hormonal changes, aging, or other underlying medical conditions.

**Why the Correct Answer is Right**
The most common clinical feature of osteoporosis is vertebral compression fractures, which can cause back pain and loss of height. This occurs when the weakened vertebral bodies collapse under normal stress, often due to the loss of trabecular bone. The vertebral bodies are the primary site of bone resorption in osteoporosis, making vertebral fractures a hallmark of the disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** Vertebral fractures are a characteristic feature of osteoporosis, but not all vertebral fractures are due to osteoporosis. Other causes include trauma, tumors, or infections.
* **Option B:** Backache can be a symptom of osteoporosis, but it is not the most common characteristic feature. Backache can also be caused by other conditions such as muscle strain or degenerative disc disease.
* **Option C:** Bowing of the legs is a characteristic feature of rickets or osteomalacia, not osteoporosis. Osteoporosis primarily affects the vertebral bodies and hips, not the long bones of the legs.
* **Option D:** Abdominal pain is not a characteristic feature of osteoporosis. While osteoporosis can cause back pain, abdominal pain is more commonly associated with other conditions such as gastrointestinal disorders or kidney stones.

**Clinical Pearl / High-Yield Fact**
Remember that osteoporosis is often asymptomatic until a fracture occurs. Therefore, early detection and treatment are crucial to prevent further bone loss and reduce the risk of fractures.

**Correct Answer: A. Fracture vertebra**