**Core Concept**
Subperiosteal erosion is a radiological finding characterized by the loss of bone beneath the periosteum, a layer of connective tissue that surrounds the bones. This process involves the resorption of bone by osteoclasts, leading to a visible break in the bone-soft tissue interface on imaging studies.

**Why the Correct Answer is Right**
Subperiosteal erosion is commonly seen in conditions characterized by increased osteoclastic activity, such as hyperparathyroidism. In this condition, excess parathyroid hormone (PTH) stimulates the release of calcium from the bones, leading to an increased resorption of bone matrix by osteoclasts. As a result, the periosteum is lifted off the bone, causing subperiosteal erosion.

In addition, subperiosteal erosion can also be seen in conditions such as metastatic bone disease, particularly from breast and lung cancers, which can cause lytic lesions that destroy bone beneath the periosteum.

**Why Each Wrong Option is Incorrect**
**Option A:** Subperiosteal new bone formation is typically seen in conditions such as osteomalacia, where there is an abnormal mineralization of bone matrix. This is the opposite of subperiosteal erosion.

**Option B:** Cortical thickening is a characteristic feature of conditions such as osteopetrosis, where there is an increase in bone density due to a defect in osteoclastic bone resorption. This is not associated with subperiosteal erosion.

**Option C:** Periosteal reaction is a nonspecific finding that can be seen in a variety of conditions, including infection, trauma, and neoplasia. While it involves the periosteum, it is distinct from subperiosteal erosion.

**Clinical Pearl / High-Yield Fact**
Subperiosteal erosion is a key radiological feature of hyperparathyroidism, and its recognition can help in the diagnosis of this condition. A high index of suspicion is essential in patients with symptoms of bone disease, particularly those with a history of hypercalcemia.

**Correct Answer:** C.