Which substance is usually absent in normal urine?

In normal urine, glucose is typically absent as it is reabsorbed by the renal tubules during the filtration process. When blood glucose levels are normal, the kidneys efficiently reclaim glucose from the filtrate back into the bloodstream. However, if blood sugar levels exceed a certain threshold, as seen in conditions like diabetes mellitus, glucose may spill into the urine, leading to glucosuria. The other substances listed—urea, creatinine, and uric acid—are normal metabolic byproducts of protein metabolism and cellular turnover, and their presence in urine is expected as they are excreted routinely by the kidneys. Thus, the correct understanding is that glucose's normal absence in urine signifies healthy renal function and metabolic control over blood sugar levels.