Which protein appears in the urine due to incomplete reabsorption caused by proximal tubular damage?

Beta2-microglobulin is a small protein that is normally filtered by the kidneys and should be reabsorbed in the proximal tubules. When there is damage to the proximal tubules, the reabsorption process becomes impaired, leading to an increased concentration of beta2-microglobulin in the urine.

In healthy individuals, beta2-microglobulin is present in very low levels in urine due to efficient reabsorption. However, in conditions causing tubular injury, such as acute tubular necrosis or other forms of renal pathology, this protein can leak into the urine. Its presence can serve as a sensitive marker for tubular dysfunction.

In contrast, urea and creatinine are not primarily affected by proximal tubular damage in terms of their filters or reabsorption; they are waste products of metabolism that are excreted based on other factors. Alpha2-macroglobulin is a larger protein that is typically not filtered through the glomerulus to any significant extent. Thus, the presence of beta2-microglobulin in urine indicates proximal tubular damage effectively, making it the correct answer.