Final answer:
The RBC abnormality often seen in healthy horses is Rouleaux formation. This condition, where red blood cells stack together, is considered normal for horses and is not an indicator of disease. The correct option is D.
Step-by-step explanation:
The RBC abnormality often seen in healthy horses is d. Rouleaux formation. This is a phenomenon where red blood cells stick together, forming stacks that resemble a row of coins.
This is not necessarily an indication of disease in horses and can be considered normal. Rouleaux can be influenced by several factors, including the proteins present in the blood plasma.
Though it is not an abnormality that causes concern in horses under normal conditions, it could indicate other issues if seen in large amounts in other species or in conjunction with other clinical signs.
It is important to differentiate between normal rouleaux formation and other RBC abnormalities such as Howell-Jolly bodies, which are nuclear remnants usually found in the RBCs of animals with certain diseases.
Heinz bodies, associated with oxidative damage inside red blood cells, and autoagglutination, a process where RBCs clump together because of the presence of antibodies, which could indicate an underlying immune-related issue. The correct option is D.