USMLE Step 1 Question of the Week #13

A 26-year-old man presents to ophthalmologist for complain of progressive loss of vision in his right eye for last six months. He has visited multiple doctors till date, but there is no improvement. There is no history of pain or redness in eyes. He often experiences fatigue for last one year. His detailed medical history reveals that he has suffered from fractures of different long bones two times; both the fractures were associated with minor injury. His detailed ophthalmologic evaluation suggests diagnosis of optic atrophy on right side. His recent complete blood counts revealed that his blood hemoglobin level was 9.2 g/dL (5.71 mmol/L). Considering a specific condition as a differential diagnosis, the ophthalmologist orders radiogram of his spine, which confirms the diagnosis. The radiogram is shown below:

Which of the following mechanisms best explains pathophysiology of the condition the man is suffering from?

A. Abnormal accumulation of mucopolysaccharides in chondrocytes

B. Decreased synthetic activity of osteoblasts

C. Primary oversecretion of parathyroid hormone

D. Defective acidification of osteoclast resorption pit

E. Defective assembly of collagen polypeptides.