Familial hyperchylomicronemia is most commonly associated with deficiency of which protein?

• A. Apolipoprotein C-II deficiency
• B. Apolipoprotein B-100 deficiency
• C. PCSK9 gain-of-function mutation
• D. LDL receptor deficiency

Answer: (A)

Familial hyperchylomicronemia arises from a problem with clearing triglyceride-rich lipoproteins after a meal. Chylomicrons, which carry dietary triglycerides, are cleared from the blood by lipoprotein lipase (LPL) in tissues, and LPL requires apolipoprotein C-II to be activated. If ApoC-II is deficient, LPL cannot properly hydrolyze the triglycerides in chylomicrons, so these particles accumulate, leading to marked hypertriglyceridemia and risk of pancreatitis. That makes apolipoprotein C-II deficiency the classic association. The other options affect LDL metabolism or VLDL/LDL assembly rather than the postprandial clearance of chylomicrons, so they don’t produce the same chylomicronemia pattern.