Which of the following is not a type of colic?

The correct answer is that black walnut colic is not a recognized type of colic. Colic, in the context of equine health, refers to abdominal pain due to various gastrointestinal issues that can arise in horses.

Gas colic occurs when gas accumulates in the intestines, leading to pain and discomfort. Impaction colic is caused by a blockage in the intestines typically from feed, sand, or foreign materials, resulting in a buildup that causes pain. Exercise-induced colic, while not as common as the other types, refers to abdominal pain that can occur as a result of strenuous physical activity.

On the other hand, black walnut colic arises from the ingestion of shavings or bedding made from black walnut wood, which can cause toxic effects leading to colic-like symptoms. It's important to differentiate that this is a reaction to a toxin rather than a classical type of colic categorized by causes related to digestive disruptions. Thus, black walnut colic doesn't fit into the standard classifications of colic types based on the underlying gastrointestinal conditions that horses typically experience.