A 3-year-old boy from rural Maharashtra presents with progressive hepatosplenomegaly, developmental delay, and recurrent respiratory infections over the past 18 months. On examination, he has coarse facial features, corneal clouding, and a palpable liver edge 4 cm below the costal margin. Bone X-rays show dysostosis multiplex. Serum enzyme assays reveal markedly elevated acid phosphatase and arylsulfatase levels. A bone marrow smear shows foamy macrophages with zebra-body inclusions on electron microscopy. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Mucopolysaccharidosis Type I (Hurler Syndrome) ### Key Diagnostic Features **Key Point:** Hurler syndrome (MPS I) is an autosomal recessive lysosomal storage disorder caused by deficiency of α-L-iduronidase, leading to accumulation of dermatan sulfate and heparan sulfate in lysosomes. ### Pathognomonic Findings in This Case 1. **Coarse facial features** — broad nose, thick lips, macroglossia, gingival hypertrophy 2. **Corneal clouding** — due to glycosaminoglycan (GAG) deposition in cornea 3. **Dysostosis multiplex** — skeletal dysplasia with vertebral beaking, kyphosis, claw hand deformity, carpal tunnel syndrome 4. **Hepatosplenomegaly** — massive, due to GAG accumulation in reticuloendothelial cells 5. **Developmental delay and neurodegeneration** — progressive CNS involvement 6. **Foamy macrophages with zebra bodies** — electron microscopy hallmark of MPS I 7. **Elevated lysosomal enzymes** — acid phosphatase, arylsulfatase (secondary elevation) ### Biochemistry Deficient enzyme: **α-L-iduronidase** Accumulated substrates: - Dermatan sulfate - Heparan sulfate Location: Lysosomes of fibroblasts, macrophages, endothelial cells, and neurons. ### Natural History | Feature | Timing | | --- | --- | | Symptom onset | 6–24 months | | Coarse features | 12–18 months | | Hepatosplenomegaly | 12–24 months | | Developmental plateau/regression | 18–36 months | | Cardiac valve disease | 2–4 years | | Death (untreated) | 4–10 years | **High-Yield:** Hurler syndrome is the most severe form of MPS I; milder variants (Scheie, Hurler-Scheie) exist but present later with less CNS involvement. ### Diagnostic Confirmation 1. **Urine GAG screening** — elevated heparan sulfate and dermatan sulfate 2. **Enzyme assay** — markedly reduced α-L-iduronidase activity in leukocytes or fibroblasts 3. **Genetic testing** — IDUA gene mutations (confirmatory) 4. **Imaging** — dysostosis multiplex on skeletal survey ### Management - **Enzyme replacement therapy (ERT):** Imiglucerase (recombinant α-L-iduronidase) — improves organomegaly and mobility but does NOT cross blood-brain barrier - **Hematopoietic stem cell transplantation (HSCT):** Gold standard if performed early (< 2 years), can halt neurodegeneration - **Supportive care:** Cardiac monitoring, hearing aids, surgical interventions (carpal tunnel, airway management) **Clinical Pearl:** Early diagnosis and HSCT before age 2 years can prevent neurological deterioration; enzyme replacement therapy alone is insufficient for CNS manifestations.