The following data were collected from a healthy volunteer before and after an intervention. Assume that plasma osmolarity and glomerular filtration rate remain constant.

                                                                                Before             After

Urine osmolarity (mOsm/L)                                 900                         250                  

Urine flow rate (ml/min)                                      0.65                        2.3

Fractional clearance of sodium                            1%                          1%

Fractional clearance of potassium                       1%                          1%

Which of the following interventions would best account for the observed changes?