An isotonic solution contains equal solute concentration on both sides.

Choice A reason: Renal failure can cause hyperkalemia because the kidneys are unable to excrete excess potassium from the body. This can lead to high levels of potassium in the blood, which can affect the heart and muscles.

Choice B reason: Diarrhea can cause hypokalemia, not hyperkalemia, because it can lead to loss of potassium from the gastrointestinal tract. This can result in low levels of potassium in the blood, which can also affect the heart and muscles.

Choice C reason: Blood transfusion can cause hyperkalemia if the blood is old or hemolyzed, meaning that the red blood cells have broken down and released potassium into the plasma. This can increase the potassium levels in the recipient's blood.

Choice D reason: Diaphoresis, or sweating, can cause hypokalemia, not hyperkalemia, because it can lead to loss of potassium from the skin. This can also lower the potassium levels in the blood.

Choice A: Elderly patients are at a higher risk for dehydration due to physiological changes that come with aging, such as decreased kidney function and physical changes to the body's water balance systems. Additionally, fever increases metabolic rate and fluid loss, and nausea and vomiting prevent adequate fluid intake, further increasing the risk of dehydration.

Choice B: While intentionally limiting fluid intake can lead to dehydration, the body's thirst mechanism in a healthy teenager is typically strong enough to prevent severe dehydration.

Choice C: Diarrhea can certainly lead to dehydration, but a young, otherwise healthy patient typically has a stronger ability to recover from fluid loss than an elderly patient.

Choice D: Infants are at a higher risk for dehydration than older children and adults due to their smaller body weight and higher turnover of water and electrolytes, but in this case, the elderly patient's multiple risk factors put them at a higher risk overall.