518. Which of the following cell types show abnormal function when there is a history in a young adult of multiple arm and leg fractures following minor falls, with a slight weakness of facial muscles on the left, mild anemia, and with generalized bony widening with partial obliteration of marrow spaces?

 A.  Granulocytic stem cells 
 B.  Megakaryocytes 
 C.  Plasma cells 
 D.  Osteoblasts 
 E.  Osteoclasts


The correct answer is E. The disease described is osteopetrosis (Albers-Schonberg disease), which
is a group of hereditary diseases in which impaired osteoclast function leads to reduced bone
resorption. The abnormal osteoclasts frequently are enlarged, with bizarre shapes. The bones
become thick and brittle; other features include anemia secondary to marrow loss and cranial nerve
deficits secondary to narrowing of bony ostea. An autosomal recessive, severe form of the disease
produces death in childhood. A relatively benign, autosomal dominant form presents in adulthood.

Abnormal proliferation of granulocytic stem cells (choice A) can produce myelocytic leukemias.

Megakaryocyte abnormalities (choice B) can produce platelet disorders, but not deficient bone
resorption.

In multiple myeloma, neoplastic plasma cells (choice C) can cause lytic bone lesions characterized
by excessive resorption of bone.

Abnormally low osteoclast, rather than osteoblast (choice D), function is the problem in osteopetrosis.