Which of the following does NOT contribute to the osmolality of plasma?

Osmolality is a measure of the concentration of solutes in a solution, and in plasma, it primarily reflects the concentration of small, water-soluble molecules. These solutes include electrolytes, glucose, urea, and other metabolites that contribute to the osmotic balance of the blood.

Lipids, while they are present in blood, are not water-soluble and do not significantly contribute to plasma osmolality. Instead, they are transported in plasma in the form of lipoproteins and do not dissociate into smaller particles that can exert osmotic pressure like electrolytes or other solutes.

In contrast, creatinine, drug metabolites, and glucose are all soluble in plasma and do contribute to osmolality. Creatinine is a waste product from muscle metabolism and is freely filtered by the kidneys, thus entering plasma. Drug metabolites are the byproducts of drug metabolism and can vary widely in their effects on osmolality. Glucose is a crucial energy source for cells and its concentration in plasma is an important aspect of metabolic regulation.

Therefore, lipids do not participate in the osmotic effects that are accounted for in plasma osmolality calculations, making them the correct option in this context.