USMLE Step 1 Question 394

Question:

A 9-month-old baby is brought to her pediatrician because she has suffered recurrent respiratory tract infections. At birth, she had coarse facial features and musculoskeletal deformities. Microscopic examination of her cells reveals large inclusion bodies due to the accumulation of undegraded products in lysosomes. An enzyme deficiency in which organelle is responsible for her disease?

Correct Answer: B.  Golgi apparatus

This patient has the clinical presentation of I cell disease, or mucolipidosis II. The disease is caused by a deficiency in N-Acglucosamine phosphotransferase in the Golgi apparatus (choice B).

This enzyme is responsible for converting specific mannose residues to mannose-6-phosphate in glycoproteins (i.e. hydrolases) destined for lysosomes. Without the enzyme, these glycoproteins follow the default pathway of constitutive exocytosis from the cell. Therefore, degradative enzymes are missing from lysosomes, and undegraded products accumulate to form the characteristic intracellular inclusion bodies.

The other answer choices are not involved in I cell disease.