Cystinuria

Cystinuria is a genetic anomaly caused by mutations in the SLC3A1 and SLC7A9 and is a condition characterized by the buildup of the amino acid Cystine, a building block of most proteins, in the kidneys and bladder. 

Chemical structure of Cystine formed from L-Cysteine                                                       (under biological conditions)

As the kidneys filter blood to create urine, Cystine is normally absorbed back into the bloodstream. Canines with cystinuria cannot properly reabsorb Cystine into their bloodstream, so the amino acid accumulates in their urine.
As urine becomes more concentrated in the kidneys, the excess Cystine forms crystals. A buildup of crystals form stones that may lodge in the kidneys or in the bladder. Sometimes Cystine crystals combine with calcium molecules in the kidneys to form large stones. These crystals and stones can create blockages in the urinary tract and reduce the ability of the kidneys to eliminate waste through urine. The stones also provide sites where bacteria may cause infections.

Cystine in the bladder can be seen as sand laying at the bottom of the bladder when at the early stage of formation. This can be seen with the help of ultrasound scanning and a skillful ultrasound operator.

Cystine crystals can also be found in urine taken for a urine test.

At a later stage, Cystine sand in the bladder forms small stones that have sharp edges and will scratch the bladder walls causing infection and blood in the urine.

Cystine urine has a strong sulfur smell and is hard to miss.