On 3 phase 99mTc-MDP bone scan, which of the following bone lesions will show least osteoblastic activity?
**Core Concept**
A bone scan using 3-phase 99mTc-MDP (technetium-99m-methyl diphosphonate) is a diagnostic imaging technique that assesses bone metabolism by detecting osteoblastic activity. Osteoblastic activity refers to the formation of new bone tissue, which is essential for bone repair and remodeling.
**Why the Correct Answer is Right**
The correct answer is A (Achondroplasia). Achondroplasia is a genetic disorder that affects bone growth and development, leading to short stature and characteristic skeletal features. In achondroplasia, there is a decrease in osteoblastic activity due to the impaired proliferation and differentiation of chondrocytes (cartilage cells), which are responsible for bone growth. As a result, the bone scan will show reduced osteoblastic activity in patients with achondroplasia.
**Why Each Wrong Option is Incorrect**
**Option B:** Osteosarcoma (a type of bone cancer) typically shows increased osteoblastic activity due to the abnormal proliferation of osteoblasts and osteoid formation. Therefore, it will not show the least osteoblastic activity.
**Option C:** Hyperparathyroidism can lead to increased osteoclastic activity and bone resorption, but it can also stimulate osteoblastic activity as the body attempts to repair the damaged bone. Thus, it may not show the least osteoblastic activity.
**Option D:** Osteoporosis is characterized by a decrease in bone density and an imbalance between bone resorption and formation. While osteoporosis may show decreased bone density, it can still have some osteoblastic activity as the body attempts to repair and form new bone tissue.
**Clinical Pearl / High-Yield Fact**
Remember that bone scans are not specific for diagnosing bone lesions and should be interpreted in conjunction with clinical findings and other imaging modalities.
**Correct Answer:** A. Achondroplasia.