You can cross out the top three answer choices, A,B,C. You wont be reabsorbing anything in the PCT in fanconi's syndrome. Looking at hypokalemia, hyponatremia, and hypophosphatemia now. Hypokalemia can't be correct because even though potassium is lost it will be reabsorbed at the later thick ascending loop and if that doesnt make sense, the body will adjust for low serum potassium but activating the H+/K+ pump on cells. It isn't hyponatremia because at the collecting duct principal cells, reabsorption will occur. This leaves hypophosphatemia as the correct and only answer choice.

This sounds like Fanconi syndrome. The proximal tubular epithelial cells have a hard time reabsorbing filtrate, so you'll see a loss of phosphate, amino acids, bicarbonate, and glucose.

The answer is actually hypokalemia, according to NBME.