This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
Paroxysmal nocturnal hemoglobinuria (PNH) is a disease that results from defects in: 
Select and explain. 
A. Ankyrin, B. Band 3, C. Spectrin, D. GPI
D: GPI
Answer: D: GPI 
Explanation: Paroxysmal nocturnal hemoglobinuria (PNH) is a disease that results from acquired mutations in the phosphatidylinositol glycan complementation group A gene (PIGA), an enzyme that is essential for the synthesis of ceain cell surface proteins. Proteins are anchored into the lipid bilayer in two ways. Most have a hydrophobic region that spans the cell membrane; these are called transmembrane proteins. The others are attached to the cell membrane through a covalent linkage to a specialized phospholipid called glycosylphosphatidylinositol (GPI). In PNH, these GPI-linked proteins are deficient because of somatic mutations that inactivate PIGA. Ref: Robbins 8th edition Chapter 14.