Osteopetrosis (marble bone disease) is primarily characterized by a functional defect in which cellular lineage?

Explanation:
Osteopetrosis is a rare genetic disorder characterized by an abnormality in osteoclast differentiation or function (often due to defects in carbonic anhydrase II). This leads to a failure of normal bone resorption and remodeling. As a result, patients develop extremely dense, sclerotic bones that are paradoxically very brittle and prone to fracture. Reference: Miller's Review of Orthopaedics, 8th Edition.