Final answer:
The likely reason for baby Jane's high blood sodium during diarrhea, given a defective SGLT1 protein, is excessive sodium reabsorption by renal tubules due to increased water reabsorption in the kidneys as a compensatory mechanism.
Step-by-step explanation:
If baby Jane's problem is a defective SGLT1 protein, the most likely reason that her blood sodium was high when she had diarrhea is related to the reabsorption of sodium by the renal tubules. Normally, SGLT1 facilitates the absorption of glucose from the gut along with sodium. When glucose is not absorbed due to a defective SGLT1, sodium absorption is also reduced in the gut. Consequently, there might be an increased reabsorption of sodium by the renal tubules to compensate for this loss, which could lead to hypernatremia (high blood sodium levels).
During diarrhea, excessive loss of water occurs, and to maintain the body's sodium and water balance, the kidneys may respond by increasing water reabsorption, which can indirectly cause an increase in sodium reabsorption. This compensatory mechanism can lead to an elevation in blood sodium concentration. Therefore, the answer is excessive sodium reabsorption by renal tubules due to increase in water reabsorption in the kidneys (option b).