Time to play! : the answer
This is the answer to our quiz Time to play! 
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See below a still image with labels. The bladder wall is normal, even ventrally (green arrows). An unusual echogenic line complies to the cranio-ventral aspect of the bladder, and considering the large amount of lipid droplets in the lumen of the bladder (some of them in yellow circles), fat suspension is suspected. The cat had no urinary signs and the presence of lipid was confirmed on urinalysis.
On the CT performed for cancer staging, a discrete hypodense line supports fat level on the non-dependent side of the bladder. See the image below (red arrows).
Lipid droplets are quite common in cats and with high resolution equipment, we see them quite well!