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Retired NBME 15 Answers

nbme15/Block 2/Question#45 (reveal difficulty score)
A 3-year-old girl is brought to the physician ...
Abnormal targeting of these enzymes to lysosomes ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ / ๐Ÿ“–
tags: biochem icell repeat

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submitted by โˆ—ergogenic22(401)
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This question describes I-cell disease (inclusion cell disease/mucolipidosis type II). You can tell because there is mixed accumulation of junk forming the inclusion bodies. Other lysosomal storage diseases have accumulation of one type of substance.

Essentially, there is a defect in defect in the enzyme N-acetylglucosaminyl-1-phosphotransferase.

This means the mannose-6-phosphate tag cannot be added to proteins. Without this tag, proteins are incorrectly sent to the extracellular space rather than delivered to lysosomes.

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ergogenic22  ^^correction. You can tell because there are elevated lysosomal enzymes in the plasma. This is because the proteins are not tagged with M6P and thus are excreted to the extracellular space. +27
sarganobhaibhand  Please Review FA2020, PageNO.47 I Cell Disease +3



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submitted by โˆ—namesthegame22(13)
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  • I-cell disease is an autosomal recessive inherited lysosomal storage disease that results from a defect in N-acetylglucosaminyl-1-phosphotransferase enzymes.

  • results in a defect of addition of mannose phosphate to lysosomal enzymes.

  • This results in the failure of mannose residue phosphorylation of lysosomal hydrolases, which subsequently leads to their abnormal expulsion from the cell instead of to their normal site of action within the lysosome.

  • Presents with coarse facial features, generalized hypotonia, bilateral hip dislocation, and inguinal hernias. Also presents with developmental delay and delayed growth.

  • increase serum activity of lysosomal enzymes.

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