Which of the following is NOT a compartment of the ruminant stomach?

The duodenum is the part of the small intestine that follows the stomach in the digestive system, specifically in both ruminants and non-ruminants. It is involved in the further digestion and absorption of nutrients after the stomach processes food.

In contrast, the compartments of the ruminant stomach consist of the rumen, reticulum, omasum, and abomasum. These four compartments work together to break down plant materials, primarily through microbial fermentation. Each compartment has a specific role; for instance, the reticulum is a site for the initial digestion and sorting of food, while the omasum absorbs water and nutrients. The abomasum is often referred to as the "true stomach," similar to that of non-ruminants, where acidic digestion occurs.

As such, the duodenum is not part of the ruminant stomach compartments, making it the correct answer to the question.