An isotonic state exists within a patient's body fluids when the solute concentration of:

Choice A reason: This is correct because 3.5 to 5.0 mEq/L is the normal range of serum potassium level in adults. Potassium is an electrolyte that is important for nerve and muscle function, as well as acid-base balance.

Choice B reason: This is incorrect because 8.5 to 10.5 mg/dL is the normal range of serum calcium level in adults, not potassium. Calcium is an electrolyte that is involved in bone health, muscle contraction, and blood clotting.

Choice C reason: This is incorrect because 135 to 145 mEq/L is the normal range of serum sodium level in adults, not potassium. Sodium is an electrolyte that is essential for fluid balance, nerve transmission, and muscle contraction.

Choice D reason: This is incorrect because 1.8 to 2.6 mEq/L is the normal range of serum magnesium level in adults, not potassium. Magnesium is an electrolyte that is important for muscle and nerve function, as well as enzyme activity.

Choice A reason: This is incorrect because intracellular fluid is greater than extracellular fluid in a hypertonic state, not an isotonic state. A hypertonic state is when the solute concentration of the extracellular fluid is higher than the intracellular fluid, which causes water to move out of the cells and shrink them.

Choice B reason: This is incorrect because extracellular fluid is less than intracellular fluid in a hypotonic state, not an isotonic state. A hypotonic state is when the solute concentration of the extracellular fluid is lower than the intracellular fluid, which causes water to move into the cells and swell them.

Choice C reason: This is correct because intracellular and extracellular fluid is equal in an isotonic state. An isotonic state is when the solute concentration of the extracellular fluid is the same as the intracellular fluid, which causes no net movement of water across the cell membrane.

Choice D reason: This is incorrect because interstitial fluid is less than the transcellular fluid in a situation of fluid imbalance, not an isotonic state. Interstitial fluid is the fluid that surrounds the cells, while transcellular fluid is the fluid that is contained in specialized cavities, such as cerebrospinal fluid, synovial fluid, or pleural fluid. The amount of transcellular fluid is normally very small compared to the interstitial fluid, but it can increase in certain conditions, such as edema, ascites, or hydrocephalus.