Final answer:
A patient with diabetes and chronic renal failure should reduce their dietary intake of Protein to decrease the excretory load on compromised kidneys. Vitamin C should also be avoided to prevent the formation of kidney stones.
Step-by-step explanation:
A patient with diabetes experiencing chronic renal failure after many episodes of renal calculi must reduce certain substances in their diet to prevent further kidney damage. Considering that chronic renal failure indicates a compromised ability to process waste substances, including the byproducts of protein metabolism, it's important to adjust the diet accordingly.
Among the options provided, Protein should be reduced to lessen the excretory load on the kidneys. A high intake of protein leads to an increased production of nitrogenous wastes, which are difficult to filter out in cases of renal insufficiency. Reducing dietary protein can help alleviate some of the strain on the kidneys' already diminished filtering capacity. While managing their condition, patients with renal failure also should avoid excess vitamin C, as it may contribute to the formation of kidney stones.
It is also crucial to control diabetes through careful management of blood glucose levels, although this does not directly answer the question about which substance to reduce in the diet due to renal failure.