Final answer:
The 42-year-old woman with systemic lupus erythematosus and renal failure is at the greatest risk for hypermagnesemia due to the kidney's impaired ability to excrete excess magnesium.
Step-by-step explanation:
The patient at greatest risk for developing hypermagnesemia is c. 42-year-old woman with systemic lupus erythematosus and renal failure. Renal failure is a critical condition that significantly impairs the kidneys' ability to excrete excess magnesium. When the kidneys are not functioning properly, as in chronic kidney disease, there is an increased risk for the accumulation of magnesium in the blood, leading to hypermagnesemia. This is supported by the understanding that patients with chronic kidney disease suffer from various metabolic imbalances, including disturbances in magnesium levels.