A child presents with developmental delay and coarsefacial features. Enzyme assay reveals a deficiency of α-L-iduronidase. Which of the following substances is most likely to accumulate in this condition?

Correct Answer: D. Dermatan sulfate + Heparan sulfate

α-L-iduronidase deficiency defines Hurler syndrome (MPS I-H), the most severe form of mucopolysaccharidosis type I. This enzyme catalyzes the first step in the degradation of both dermatan sulfate and heparan sulfate by cleaving the terminal α-L-iduronic acid residues from these glycosaminoglycans (GAGs). When the enzyme is deficient, both substrates accumulate simultaneously in lysosomes, leading to the characteristic clinical triad: developmental delay, coarse facial features (broad nose, thick lips, frontal bossing), and hepatosplenomegaly. The accumulation occurs in multiple tissues—brain, heart, bones, and connective tissue—explaining the multisystem involvement. Chondroitin sulfate is degraded by different enzymes (β-glucuronidase and other sulfatases) and does not accumulate in MPS I. The key discriminator is that α-L-iduronidase acts on the terminal iduronic acid of both dermatan and heparan sulfate, making both substrates accumulate in equal measure. This is distinct from other MPS types where single GAG accumulation occurs (e.g., MPS II with heparan sulfate alone, MPS IV with chondroitin sulfate and keratan sulfate).

Why the other options are wrong

A. Only Dermatan sulfate — This is wrong because α-L-iduronidase degrades BOTH dermatan and heparan sulfate equally. Selecting only dermatan sulfate misses the dual substrate specificity of this enzyme. This trap lures students who memorize single GAG accumulation patterns without understanding enzyme substrate specificity. MPS I-H always shows accumulation of both GAGs. B. Heparan sulfate + Chondroitin sulfate — This is wrong because chondroitin sulfate is NOT a substrate of α-L-iduronidase; it is degraded by different enzymes (β-glucuronidase, N-acetylgalactosamine-4-sulfatase). This option confuses MPS I with MPS IV (Morquio syndrome), where chondroitin sulfate and keratan sulfate accumulate. The NBE trap pairs heparan sulfate (correct) with an incorrect GAG. C. Dermatan sulfate + Chondroitin sulfate — This is wrong because chondroitin sulfate does not accumulate in MPS I-H; it is a substrate for different lysosomal enzymes. This option again conflates MPS I with MPS IV. Students who confuse the enzyme substrate specificity or mix up different MPS types fall into this trap. Dermatan sulfate alone is insufficient for the diagnosis.

High-Yield Facts

Mnemonics

MPS I = Iduronidase → Iduric acid cleavage → Iduran + Heparan α-L-iduronidase cleaves iduronic acid from both dermatan (formerly called chondroitin sulfate B) and heparan sulfate. The 'I' links enzyme, substrate, and both GAGs. Use when you see 'iduronidase' in the stem. Hurler = Heparan + Hyaluronic (dermatan) = Heavy accumulation Hurler syndrome (MPS I-H) causes dual GAG accumulation. Remember: Hurler has Heparan + dermatan (historically called hyaluronic). Contrast with Hunter (MPS II, X-linked, heparan alone).

NBE Trap

NBE pairs α-L-iduronidase with single GAG accumulation (option A) or mixes in chondroitin sulfate (options B, C) to test whether students understand that this enzyme has dual substrate specificity for both dermatan and heparan sulfate, not one or three GAGs.

Clinical Pearl

In Indian pediatric practice, Hurler syndrome presents by 6–12 months with developmental regression, coarse features, and hepatosplenomegaly. Early recognition and referral for HSCT before age 2 years is critical—delayed diagnosis results in progressive neurodegeneration and death by age 5–8 years without intervention. Enzyme assay confirming α-L-iduronidase deficiency with dual GAG accumulation is the diagnostic gold standard.

_Reference: OP Ghai Essentials of Pediatrics Ch. 10 (Inborn Errors of Metabolism); Robbins Pathologic Basis of Disease Ch. 5 (Genetic Disorders)_