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517. A 5-year-old child has blue-tinged sclera, hearing loss, and small, slightly blue, misshapen teeth. Radiologic studies confirm the presence of numerous fractures of various ages. No significant degree of bruising is seen over sites of recent fracture. The disease this child most likely has is related to abnormal metabolism involving which of the following substances?


 A.  Collagen 
 B.  Glycogen 
 C.  Mucopolysaccharides 
 D. Purines 
 E.  Tyrosine 

The correct answer is A. The suspected disease is osteogenesis imperfecta, which is a rare genetic
disorder that occurs in both recessive and dominant forms. The clinical presentation, depending on
the specific form, varies from death in utero, to that described in the question stem, to very mild
disease with only a modest increase in bone fragility. Spontaneous fractures occur in utero or during
childhood. The different types all have defects in the synthesis of type I collagen, often with
insufficient or abnormal pro-1(1) or pro-2(1) chains. These deficits produce an unstable collagen triple
helix that is not as strong as normal collagen. Less severe mutations on type I collagen genes are
common, resulting in collagen disarray and predisposing to hypogonadal or idiopathic osteoporosis.

Defective glycogen (choice B) metabolism is associated with the various glycogen storage diseases,
such as von Gierke disease and Pompe disease. These diseases tend to present with profound
hypoglycemia, hepatomegaly, or muscle weakness. 

Defective mucopolysaccharide (choice C) metabolism is associated with the mucopolysaccharidoses,
such as Hurler and Hunter syndromes. These  diseases tend to present with abnormal facies
("gargoylism"), deformed ("gibbus") back, claw hand, and stiff joints. 

Abnormalities of purine metabolism (choice D) are present in gout, which presents with joint
inflammation and often involves the great toe. The net result is due to chronic hyperuricemia.

Abnormalities of tyrosine metabolism (choice E) are associated with phenylketonuria (pale hair and
skin, mental retardation, musty smelling urine), albinism (pale hair, skin, increased skin cancer),
cretinism (decreased T3 and T4), tyrosinosis (liver and kidney disease), and alkaptonuria (chronic
arthritis and urine that turns black upon standing).

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