Final answer:
A trace amount of bilirubin in the urine is considered abnormal in equines, as horses typically do not have bilirubin in their urine due to distinct liver functions and absence of a gallbladder.
Step-by-step explanation:
In which species is even a trace amount of bilirubin in the urine considered abnormal and a sign of disease? The correct answer is equine (b.). In horses, even small amounts of bilirubin in the urine are considered abnormal, unlike in other species where a small amount might be normally present. This is because horses have a unique physiology where their livers continuously secrete bile directly into the intestine, rather than being stored in the gallbladder, as they lack this organ. As a result, bilirubin is not typically present in horse urine unless there is some form of liver damage or another pathological condition causing hemolysis, leading to an overflow of bilirubin into the urine.
Bilirubin is a breakdown product of hemoglobin from old or damaged red blood cells. The spleen removes these cells, and the liver processes the bilirubin. It's secreted into bile and then converted by intestinal bacteria into a brown pigment called stercobilin, which colors feces. When the liver cannot effectively remove bilirubin from circulation due to disease, it may lead to jaundice. However, the presence of bilirubin in the urine of horses always warrants further investigation.