What is the primary substance stored by the gallbladder?

The gallbladder primarily stores bile, which is a digestive fluid produced by the liver. Bile contains bile salts, bilirubin, cholesterol, and other components, and its main function is to aid in the digestion and absorption of fats in the small intestine. When food enters the digestive tract, especially fatty foods, the gallbladder releases bile into the duodenum, enhancing fat digestion and absorption.

Understanding the role of the gallbladder highlights why bile is the correct answer, as it plays a crucial part in the digestive process, in contrast to other substances listed. Cholesterol is a component of bile but is not the primary substance stored. Insulin is produced by the pancreas, not stored in the gallbladder, and glycogen is a form of energy storage found mainly in the liver and muscles, unrelated to the gallbladder's functions.