Metabolism USMLE Step 1 Practice Question
A 5-year-old male with growth retardation and mild intellectual impairment has an enlarged thyroid. His serum thyroid-stimulating hormone (TSH) level is 4.3 mU/L (normal: 0.1-4.0 mU/L), and his serum thyroglobulin level is <15 pmol/L (normal: 15-50 pmol/L). A biopsy of the thyroid stained with antithyroglobulin antibodies reveals accumulation of thyroglobulin in the rough endoplasmic reticulum. Which of the following underlying defects is most likely in this male?
Answer choices
- ALoss of function mutation in the DNA encoding the hydrophobic signal sequence on thyroglobulin
- BLoss of function mutation in the ubiquitin gene
- CThyroglobulin monomer misfolding and failure of subunit assemblyCorrect answer
- DFailure to phosphorylate mannose attached to thyroglobulin
- EA TSH receptor defect
- FFailure of thyroglobulin sulfation in the Golgi apparatus before export to the follicular lumen
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