Which metabolic compound consists only of carbon, hydrogen, and oxygen?

The compound that consists only of carbon, hydrogen, and oxygen is glucose. Glucose is a simple sugar, or monosaccharide, with the chemical formula C6H12O6, which clearly shows that it contains only these three elements.

Starch, cellulose, and glycogen are all polysaccharides composed of many glucose molecules linked together, but they also consist of the same elemental components of carbon, hydrogen, and oxygen in a complex arrangement. However, glucose is the only one that is classified as a simple metabolic compound in this question, directly showcasing its composition of just those three elements without being bonded into a larger structure.

Thus, when considering the question's criteria for the simplest form that fits the definition provided, glucose stands out as the correct answer.