Which apolipoprotein serves as a surrogate marker for HDL?

• A. Apo A1
• B. Apo B
• C. Apo C
• D. Apo E

Answer: (A)

HDL's major apolipoprotein is Apo A-I, which is essential for HDL formation and its role in reverse cholesterol transport. Because most HDL particles contain Apo A-I, measuring Apo A-I levels reflects the quantity of HDL particles and, to a good extent, the HDL cholesterol content. This makes Apo A-I a convenient surrogate marker for HDL.

Apo B represents the apolipoprotein component of atherogenic particles like LDL and VLDL, so it indicates the number of potentially harmful particles rather than HDL. Apo C is a cofactor found on several lipoproteins and does not serve as a specific proxy for HDL. Apo E appears on various particles too and is not a reliable indicator of HDL levels.