A blood sample agglutinates with anti-A, anti-B, and anti-H sera. The serum does not agglutinate any reagent red cells including group O cells. This patient has a rare phenotype in which even genetically A or B individuals type as group O because the precursor substance is absent. Which enzyme, encoded by the H gene, is missing in this Bombay (Oh) phenotype?

• A L-fucosyltransferase (H transferase) ✓
• B Galactosyltransferase (B transferase)
• C N-acetylgalactosaminyltransferase (A transferase)
• D Glucuronyltransferase

Explanation

The Bombay phenotype results from homozygous recessive hh genotype lacking L-fucosyltransferase (H transferase), which normally adds L-fucose to the precursor chain to form H antigen. Without H antigen, C and B transferases cannot synthesize C or B antigens, so the individual types as O despite carrying C or B genes. Options C and B are the enzymes for C and B antigens respectively, which may be genetically present but functionally inactive.

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.