Or an adrenal gland. Don't forget that.
Shortly after getting out into practice I had a non-specific gastoenteritis dog - 3 months old - that the doc before me admitted and I took on rounding during my shift. It was one of those that just felt a bit odd, so I did a quick scan - something between an aFAST and a "real" abdominal ultrasound. Found this weird big cyst-like thing in the cranial abdomen. Maybe 8-9cm?
Stared and stared. Tried to decide what the heck that could be. Couldn't decide. Maybe pancreatic cyst, but in a 3 month old? Dog did great, but I sent it off to IntMed for a real ultrasound - very committed owners, and I told them "this is weird".
IntMed did their scan and said "yup, this is weird. Dunno. Maybe pancreatic cyst. Recheck in a month since the dog is doing great."
Morale: Trust yourself. What you see on ultrasound probably is what you think it is.