**Core Concept**
The 'onion peel' appearance refers to a characteristic radiographic pattern observed in certain conditions, where alternating layers of increased and decreased density are seen, resembling the layers of an onion. This pattern is typically associated with conditions involving the deposition of calcium or other substances in the bone.

**Why the Correct Answer is Right**
The 'onion peel' appearance is most commonly seen in the context of Paget's disease of bone, a condition characterized by an abnormal breakdown and regrowth of bone tissue. This leads to a disorganized and irregular bone structure, with alternating layers of dense and porous bone. The deposition of calcium and other minerals in these layers contributes to the characteristic 'onion peel' appearance on radiographs. The abnormal bone resorption and formation in Paget's disease result in an increased bone turnover, which can be visualized on imaging studies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the 'onion peel' appearance is not typically associated with osteoporosis, which is characterized by a generalized decrease in bone density and is not known for the deposition of calcium or other substances in the bone.

**Option B:** This option is incorrect because the 'onion peel' appearance is not typically associated with fibrous dysplasia, which is characterized by the replacement of normal bone with fibrous tissue and does not typically result in the deposition of calcium or other substances in the bone.

**Option C:** This option is incorrect because the 'onion peel' appearance is not typically associated with osteomalacia, which is characterized by a softening of the bones due to a deficiency of vitamin D or calcium, and does not typically result in the deposition of calcium or other substances in the bone.

**Clinical Pearl / High-Yield Fact**
The 'onion peel' appearance is a classic radiographic finding in Paget's disease of bone, and it is essential for clinicians to be aware of this characteristic pattern to make an accurate diagnosis. This pattern can be seen in various imaging modalities, including plain radiographs, CT scans, and MRI.

**Correct Answer:** C.