**Core Concept**
Paget disease is a chronic bone disorder characterized by abnormal bone remodeling, leading to bone deformities and an increased risk of osteosarcoma. It results from an imbalance in the bone remodeling process, where osteoclasts excessively resorb bone, followed by an overactive osteoblastic response, leading to disorganized bone formation.

**Why the Correct Answer is Right**
The correct answer, **Option D**, is incorrect because Paget disease typically results in cortical bone thickening, not thinning. The disease process involves an initial phase of osteoclastic bone resorption, followed by a phase of osteoblastic bone formation. This leads to a characteristic "cotton wool" appearance on radiographs, with thickened, disorganized bone that can cause deformities and an increased risk of fractures.

**Why Each Wrong Option is Incorrect**
**Option A:** Paget disease can occur at any age, but the peak incidence is between 50 and 80 years, making this statement true, not false.
**Option B:** Paget disease can indeed be either monostotic (affecting a single bone) or polyostotic (affecting multiple bones), so this statement is also true.
**Option C:** The hallmark of Paget disease is an imbalance between osteoblastic and osteoclastic activity, with excessive osteoclastic bone resorption followed by an overactive osteoblastic response, making this statement true.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Paget disease can be a precursor to osteosarcoma, and patients with Paget disease should be carefully monitored for signs of malignant transformation.

**✓ Correct Answer: D. Deformed long bones result from cortical bone thinning**