Hyperlipidemia

A 30-year-old woman comes to the emergency department due to severe epigastric pain that radiates to her back accompanied by nausea and vomiting. She denies consuming alcohol within the past week. Physical examination shows multiple soft, orange papules on the right posterior elbow. Serum studies show leukocytosis and elevated levels of amylase and lipase. One of the laboratory technicians reports that a white, milky layer formed on top of the sample after centrifugation. Which of the following proteins is most likely deficient in this patient?