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NBME 15 Answers

nbme15/Block 2/Question#45

A 3-year-old girl is brought to the physician ...

Abnormal targeting of these enzymes to lysosomes

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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.

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. +