A child with coarse facial features and developmental delay is found to have a deficiency of α-L-iduronidase on enzyme testing. Which substances are expected to accumulate in this disorder?

Correct Answer: C. Dermatan sulfate + Heparan sulfate

α-L-iduronidase deficiency defines Hurler syndrome (MPS I-H), the most severe form of mucopolysaccharidosis type I. This lysosomal enzyme catalyzes the cleavage of α-L-iduronic acid residues from glycosaminoglycans (GAGs). When deficient, two specific substrates accumulate: dermatan sulfate and heparan sulfate. These are the only two GAGs that contain α-L-iduronic acid in their structure and therefore require this enzyme for their degradation. Chondroitin sulfate, while a GAG, does not contain α-L-iduronic acid and is not a direct substrate for α-L-iduronidase. The clinical presentation—coarse facial features, developmental delay, hepatosplenomegaly, and skeletal dysostosis—results from lysosomal accumulation of these two GAGs in connective tissue, bone, and central nervous system. In Indian pediatric practice, Hurler syndrome presents with progressive neurodegeneration and typically manifests by 6–12 months of age. The enzyme deficiency is confirmable by leukocyte enzyme assay, and substrate accumulation can be detected in urine as elevated GAG levels. Hematopoietic stem cell transplantation (HSCT) is the only disease-modifying therapy available in India for eligible candidates.

Why the other options are wrong

A. Only Dermatan sulfate — This is wrong because α-L-iduronidase degrades both dermatan sulfate and heparan sulfate—it is not substrate-specific to dermatan sulfate alone. Heparan sulfate also contains α-L-iduronic acid residues and accumulates equally in Hurler syndrome. Selecting only one substrate misses the biochemical reality of the enzyme's dual substrate specificity and would be incomplete pathophysiology. B. Heparan sulfate + Chondroitin sulfate — This is wrong because chondroitin sulfate does not contain α-L-iduronic acid in its structure; it is composed of N-acetylgalactosamine and glucuronic acid. Therefore, chondroitin sulfate is not a substrate for α-L-iduronidase and does not accumulate in MPS I-H. This option represents a common NBE trap pairing two GAGs without considering their actual chemical composition and enzyme specificity. D. Dermatan sulfate + Chondroitin sulfate — This is wrong because chondroitin sulfate is not a substrate for α-L-iduronidase (it lacks α-L-iduronic acid residues). While dermatan sulfate correctly accumulates, pairing it with chondroitin sulfate reflects confusion about GAG biochemistry. The trap here is that both are GAGs, but only those containing α-L-iduronic acid accumulate in this specific enzyme deficiency.

High-Yield Facts

Mnemonics

MPS I Substrates: 'DH' (Dermatan + Heparan) α-L-iduronidase breaks down Dermatan and Heparan sulfate. Both contain α-L-iduronic acid. Chondroitin sulfate (C) is NOT a substrate—it has glucuronic acid instead. Use 'DH' to remember the two GAGs that accumulate in Hurler syndrome. Hurler = Coarse + Delay + Hepato Hurler = Heparan/Dermatan accumulation → Hepatomegaly, coarse features, developmental delay. The 'H' links the enzyme deficiency to the clinical triad.

NBE Trap

NBE pairs "heparan sulfate + chondroitin sulfate" (option B) to exploit students who know that multiple GAGs accumulate in MPS but confuse which GAGs are actually substrates for α-L-iduronidase. The trap relies on recognizing that chondroitin sulfate, while a GAG, lacks the α-L-iduronic acid residues required for this enzyme's action.

Clinical Pearl

In Indian pediatric practice, a child presenting with coarse facies, developmental regression, and hepatosplenomegaly by 6–12 months should raise immediate suspicion for Hurler syndrome. Early diagnosis via enzyme assay and genetic counseling are critical because HSCT outcomes are best when performed before irreversible CNS damage occurs—typically before age 2 years.

_Reference: OP Ghai Pediatrics Ch. 9 (Inborn Errors of Metabolism); Robbins Ch. 5 (Genetic Disorders)_