**Core Concept**
Looser's zones, also known as pseudofractures, are radiolucent bands or bands of decreased bone density that occur in the bones of patients with osteomalacia or rickets. These lesions are due to the accumulation of unmineralized osteoid tissue, which is a result of impaired bone mineralization.

**Why the Correct Answer is Right**
Looser's zones are typically found in the weight-bearing bones of the lower limbs, such as the femur, pelvis, and tibia, as well as in the ribs and scapulae. They are caused by the failure of osteoblasts to mineralize osteoid tissue, leading to the formation of uncalcified bone. This is often due to a deficiency of vitamin D, calcium, or phosphate, or due to impaired renal function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Looser's zones are commonly seen in the weight-bearing bones of the lower limbs, which include the femur, pelvis, and tibia. The proximal femur is a classic location for Looser's zones.

**Option B:** This option is incorrect because Looser's zones are not typically seen in the diaphysis (shaft) of long bones. Instead, they are more commonly found in the metaphysis (growth plate area) or in the cortical bone adjacent to the growth plate.

**Option C:** This option is incorrect because Looser's zones are not typically seen in the vertebrae. While osteomalacia can cause vertebral compression fractures, Looser's zones are not a characteristic feature of vertebral involvement.

**Clinical Pearl / High-Yield Fact**
Looser's zones are a classic radiographic finding in osteomalacia, but they can also be seen in other conditions, such as Paget's disease and hyperparathyroidism. Remember that Looser's zones are a sign of impaired bone mineralization, which can occur due to a variety of causes, including vitamin D deficiency, renal failure, and certain medications.

**Correct Answer: D. Vertebrae.**