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The reason large molecules such as blood and plasma proteins cannot enter the kidney tubules during filtration primarily hinges on the structural characteristics of the filtration membrane within the nephrons of the kidneys. The glomerular filtration barrier is comprised of three layers: the endothelial cells of the glomerular capillaries, the basement membrane, and the podocytes that surround the capillaries. This barrier is specifically designed to allow small molecules, such as water, ions, and small organic molecules, to pass through while restricting larger molecules.

Large molecules, including blood and plasma proteins, are typically too big to fit through the small pores found in this permeable membrane. This selective permeability is crucial for maintaining the balance of fluids and electrolytes in the body, preventing the loss of essential proteins and cells that play critical roles in various physiological functions.

Other options do not accurately address the mechanism preventing the entry of large molecules into the kidney tubules. For instance, the composition of urine, which does contain waste products but is not solely urea and water, does not explain the filtration process. Similarly, while blood and plasma proteins are indeed vital for the body, this fact does not physically prevent their filtration. Lastly, the statement that the cell membrane of the capill