Many substances are present in proximal tubular fluid in the kidney, but which one is NOT normally present in urine?

Protein (albumin)

Explanation:

The presence of albumin in the urine indicates that the glomerulus filtering system of the kidney is damaged and is not working properly, as it's allowing large molecules to pass through. Protein in urine is a sign of kidney disease, as the kidney filters the blood and all the good stuff, proteins being one of them, is reabsorbed back into the blood.

Filtration of blood takes place in the glomerulus of the kidneys, where all the toxins and bad stuff (waste) are filtered through the glomerulus and taken out of the body, but large molecules such as protein molecules and blood cells cannot pass through the pores and remain in the blood, so if urine contains albumin or blood cells it means that the glomerulus has been damaged. If any water, amino acids, glucose or potassium are lost during glomerular filtration they will be reabsorbed and re-enter back into the blood in the proximal convoluted tubule of the nephron. So there shouldn't be any protein in urine. Diabetes and high blood pressure can also cause kidney disease and the glomerular filter does not work well, one will start spilling all the good things, then there will be sugar and proteins present in the urine. So protein in urine indicates kidney disease.