Inherited, diffuse bone disease that results in increased sclerotic appearance of the skeleton on radiological examination (from Latin: "petrosus" = stony).

Etiology:- Type I osteopetrosis (malignant osteopetrosis)→ Age of onset: infancy→ Clinical course: severe- Type II osteopetrosis (benign osteopetrosis, Albers-Schonberg disease)→ Age of onset: early adulthood→ Clinical course: mild

Pathophysiology: impaired osteoclastic resorption of bone with preserved osteoblastic function

Clinical features:- Pathological fractures- Cranial nerve disorders- Pancytopenia- Hepatosplenomegaly

Diagnostics:- X-ray: homogenous, marbled thickening of both cortical and trabecular bone- Laboratory findings: Hypocalcemia, ↑ tartrate-resistant acid phosphatase (TRAP)

Therapy: Bone marrow transplantation may be curative as osteoclasts are derived from monocytes.