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    BioPatel is offline Member
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    Fraternal twins have episodes of fasting hypoglycemia

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    Fraternal twins have episodes of fasting hypoglycemia, hypoketonemia, and muscle weakness occurring only during periods of caloric deprivation. One twin dies at 5 months of age. The surviving twin develops a cardiomyopathy, which is diagnosed at 3 years of age. Carnitine esters are elevated in muscle tissue and in serum. Which of the following is the most likely diagnosis?

    A. α-L-Iduronidase deficiency
    B. Acid maltase deficiency (α 1,4-glucosidase deficiency)
    C. Carnitine uptake deficiency
    D. Long-chain acyl CoA dehydrogenase (LCAD) deficiency
    E. Muscle glycogen phosphorylase deficiency

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    Asclepius1's Avatar
    Asclepius1 is offline Ultimate Member 537 points
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    is it B?

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    BioPatel is offline Member
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    The correct answer is D. Each of the genetic deficiencies listed may present with cardiomyopathy in childhood. LCAD affects the first step in β-oxidation of fatty acids in the mitochondria. Cardiac muscle and resting skeletal muscle use fatty acids and ketones as a primary energy source, thus in LCAD deficiency, muscle weakness and eventually cardiomyopathy develop. Fasting hypoglycemia and hypoketosis occurring together are strongly suggestive of a block in fatty acid oxidation. Only a carnitine uptake deficiency (choice C) or LCAD deficiency (choice D) are related to fatty acid oxidation. Since carnitine esters (long chain acyl carnitines) are accumulating in serum and in muscle tissue, a carnitine uptake deficiency is ruled out.

    α-L-Iduronidase deficiency, or Hurler disease (choice A) is another of the lysosomal storage diseases. Cardiomyopathy, valvular defects, and potential occlusion of coronary arteries, are results of specific glycosaminoglycan (heparan sulfate and dermatan sulfate) accumulation. Similar substrate accumulation in other tissues results in corneal clouding, mental retardation, and skeletal abnormalities. Hypoglycemia, hypoketosis, and muscle weakness are not prominent features of Hurler disease. There would be no accumulation of carnitine derivatives.

    Acid maltase is a lysosomal enzyme, and its deficiency produces one of a host of lysosomal storage diseases. Acid maltase deficiency (α 1,4-glucosidase deficiency; choice B) or Pompe disease is also associated with cardiomyopathy. In this case glycogen fragments accumulate in the lysosomes, primarily affecting tissues that store glycogen (muscle, liver, kidney). Hypoglycemia and hypoketosis are not features of this deficiency, nor would acyl carnitines accumulate.

    Muscle glycogen phosphorylase deficiency (choice E), or McArdle disease, is also associated with muscle weakness and potential cardiomyopathy, but there is no associated hypoglycemia or hypoketosis. Muscle glycogen is not released as free glucose into the blood, and ketone synthesis is a hepatic pathway associated with fatty acid oxidation in that tissue. A muscle biopsy of a patient with muscle glycogen phosphorylase deficiency would reveal glycogen deposits. There would be no increase in acyl carnitines.

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    Asclepius1's Avatar
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    My answer is D. Long-chain acyl CoA dehydrogenase (LCAD) deficiency

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    Fatty Acid Synthesis

    FATTY ACID SYNTHESIS



    Stored fuel is normally present as fat, but the diet is mostly
    carbohydrate.
    Thus, there must be conversion of
    carbohydrate to fat
    . Note: This process cannot be
    reversed
    since AcCoA cannot be made into carbohydrate.

    Any compound metabolized to acetyl CoA can be a precursor for
    fat synthesis
    (Fig. 9). In liver and adipose tissue, glucose
    is the primary precursor for fatty acid synthesis. Fatty acid
    biosynthesis occurs in the cytoplasm and primarily in liver cells

    (adipose tissue as well).








    Figure 9 - Transfer of acetyl CoA from mitochondria to
    cytosol
    (see also Stryer Fig 24-27).

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