A 6-month-old infant presents with failure to thrive, obvious developmental delay, abnormal skeletal development, coarse facial features, and restricted joint movement. Cellular and biochemical analysis indicates the infant is suffering from a defect in protein modification. These clinical symptoms are most indicative of which of the following disorders?

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Quizplus · Accepted Answer

The symptoms described are characteristic of I-cell disease, which is caused by a defect in the modification of lysosomal enzymes, leading to their improper targeting and accumulation of undegraded substrates.  I-cell disease (also called mucolipidosis II α/β, ML-II α/β) is an autosomal recessive disorder that results as a consequence of defective targeting of lysosomal hydrolases to the lysosomes. The disorder is so called because fibroblasts from afflicted patients contain numerous phase-dense inclusion bodies in the cytosol. I-cell disease is characterized by severe psychomotor retardation that rapidly progresses. leading to death between 5 and 8 years of age. I-cell patients exhibit coarse facial features, craniofacial abnormalities, and severe skeletal abnormalities. Additional clinical symptoms of I-cell disease include hepatomegaly, cardiomegaly, umbilical hernias, and recurrent upper respiratory infections.

A 6-Month-Old Infant Presents with Failure to Thrive, Obvious Developmental