Which of the following is NOT primarily a protein-based biological molecule?

• A. Steroid hormones such as cortisol, which are synthesized from cholesterol and act through intracellular receptors rather than being composed of amino acids
• B. Hemoglobin, a tetrameric protein that transports oxygen in red blood cells
• C. Many enzymes, which are protein catalysts that increase reaction rates in metabolism
• D. Insulin, a peptide hormone that is synthesized as a protein and regulates blood glucose

Answer: (A)