A 3-year-old boy has repeated bacterial and viral infections. Blood counts show normal neutrophil numbers. Neutrophil function studies reveal normal oxidative burst and normal chemotaxis once cells reach the site, but flow cytometry shows absence of sialyl-Lewis X (CD15s) expression. The child's red cell typing reveals the Bombay blood group phenotype. Which enzyme step is defective?

• A Beta-2 integrin chain synthesis
• B GDP-fucose transport into the Golgi ✓
• C Myeloperoxidase-dependent killing
• D NADPH oxidase assembly

Explanation

This is leukocyte adhesion deficiency type II, caused by defective GDP-fucose transport into the Golgi, impairing fucosylation of glycans including sialyl-Lewis X, the ligand for endothelial selectins. Without it, neutrophil rolling cannot occur. The associated Bombay phenotype (lack of H antigen, itself fucose dependent) is the classic diagnostic clue. Beta-2 integrin defects define LAD-I, and both oxidase options describe defects seen in chronic granulomatous disease, not here.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.