The very high density of the gallbladder contents indicates the presence of either calcium or iodine.
In patients with renal insufficiency, intravenously administered contrast media are often excreted through the biliary system and may get stuck in the gallbladder for a while.
In this case, no contrast media had been given recently. As such, calcium milk is the most probable diagnosis.
Although the ultrasound was negative on that part, the presence of one or more small obstructive non-radiopaque gallstones in the cystic or choledochus ducts can't be formally excluded, yet could account for the gallbladder hydrops.
On the other hand, the hydrops may be reflecting a long-standing dysfunctional gallbladder due to chronic cholecystitis, even in the absence of biliary stones.