**Core Concept**
Looser's zones refer to a specific radiographic finding characterized by a zone of trabecular bone resorption, which is often seen in conditions associated with osteomalacia or rickets. This phenomenon is a result of the imbalance between bone resorption and bone formation, leading to the weakening of the bone structure.

**Why the Correct Answer is Right**
Looser's zones are typically observed in osteomalacia, a condition where there is a deficiency of vitamin D, leading to impaired bone mineralization. This results in an increased activity of osteoclasts, which are the bone-resorbing cells, and a subsequent decrease in bone density. The characteristic appearance of Looser's zones on radiographs is due to the resorption of trabecular bone, creating a distinct 'picture frame' or 'rugger jersey' appearance. This is a hallmark feature of osteomalacia and is often used as a diagnostic clue.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Looser's zones are not typically associated with osteoporosis, which is characterized by a generalized decrease in bone density without any specific radiographic features.

**Option B:** This option is also incorrect because Looser's zones are not characteristic of Paget's disease, which is a condition of abnormal bone remodeling, resulting in an increase in bone density.

**Option C:** This option is incorrect because Looser's zones are not typically seen in fibrous dysplasia, which is a condition characterized by the presence of fibrous tissue within the bone.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Looser's zones are often seen in patients with a history of malabsorption, particularly those with gastrointestinal disorders, as these conditions can lead to a deficiency of vitamin D and calcium.

**Correct Answer: D. Osteomalacia**