There are some people who are not too familiar with hydrophilic drugs. This type of drugs usually stays in the blood until the extra fluid becomes eliminated. This is usually eliminated through renal extraction.
This explains why there is an increase in the volume distribution. Since this soluble in water, this is not absorbed by the rest of the body. Rather, this is going to be excreted. The best option is for the patient to take hydrophobic drugs so that the volume will become higher.
The patient was taking a hydrophillic drug. with the onset of edema, more plasma fluid would enter the interstitial spaces. this increases the volume of distribution of a hydrophillic drug that was only confined to the plasma