An isotonic solution contains equal solute concentration on both sides.

Choice A reason: This is incorrect because low HCO3- and high PaCO2 are signs of metabolic acidosis, not alkalosis. Metabolic acidosis is a condition where the blood pH is lower than the normal range of 7.35 to 7.45, due to an excess of acids or a loss of bases in the body.

Choice B reason: This is incorrect because low PaCO2 and low HCO3- are signs of respiratory acidosis, not alkalosis. Respiratory acidosis is a condition where the blood pH is lower than the normal range of 7.35 to 7.45, due to impaired gas exchange or hypoventilation, which causes carbon dioxide to accumulate in the blood.

Choice C reason: This is correct because low PaCO2 and high HCO3- are signs of alkalosis. Alkalosis is a condition where the blood pH is higher than the normal range of 7.35 to 7.45, due to a loss of acids or an excess of bases in the body. There are two types of alkalosis: respiratory and metabolic. Respiratory alkalosis is caused by hyperventilation, which lowers the PaCO2 in the blood. Metabolic alkalosis is caused by vomiting, diuretics, or excessive intake of antacids, which raises the HCO3- in the blood.

Choice D reason: This is incorrect because high PaCO2 and high HCO3- are signs of compensation, not alkalosis. Compensation is a process where the body tries to restore the normal pH by adjusting the levels of PaCO2 and HCO3- in the opposite direction of the primary disorder. For example, if the patient has metabolic alkalosis, the respiratory system will try to compensate by retaining carbon dioxide and lowering the PaCO2. If the patient has respiratory alkalosis, the renal system will try to compensate by excreting bicarbonate and lowering the HCO3-.

Choice A reason: Intracellular to the extracellular fluid shift is not the cause of low blood pressure in this case. This fluid shift occurs when the cells lose water due to osmosis, such as in dehydration or hypernatremia.

Choice B reason: Interstitial to intravascular fluid shift is not the cause of low blood pressure in this case. This fluid shift occurs when the fluid moves from the tissue spaces to the blood vessels, such as in hypovolemia or hypotonic fluid administration.

Choice C reason: Interstitial to the intracellular fluid shift is not the cause of low blood pressure in this case. This fluid shift occurs when the fluid moves from the tissue spaces to the cells, such as in overhydration or hyponatremia.

Choice D reason: Intravascular to the interstitial fluid shift is the cause of low blood pressure in this case. This fluid shift occurs when the fluid moves from the blood vessels to the tissue spaces, such as in edema, inflammation, or increased capillary permeability. This reduces the blood volume and lowers the blood pressure.