Which protein is the primary carrier of retinol (Vitamin A) in serum?

• A. Retinol Binding Protein
• B. Transthyretin
• C. Albumin
• D. Transferrin

Answer: (A)

In serum, vitamin A (retinol) needs a dedicated carrier because it is not water-soluble. The main transporter is retinol-binding protein, a liver-made protein that binds retinol tightly and ferries it through the bloodstream to target tissues. In circulation, the RBP–retinol complex often associates with transthyretin, forming a larger complex that helps prevent renal clearance and stabilizes delivery, but the primary carrier responsible for delivering retinol is the retinol-binding protein itself. Albumin handles many small molecules and fatty acids, not retinol specifically, while transferrin carries iron. Therefore, the best answer is retinol-binding protein.