Which of the following is NOT a part of the digestive system?

The kidney is not a part of the digestive system; instead, it is a crucial component of the urinary system. The digestive system is responsible for the breakdown of food, nutrient absorption, and waste elimination related to the gastrointestinal tract, which includes organs such as the esophagus, stomach, liver, intestines, and others that facilitate the digestion and processing of food.

The esophagus is a muscular tube that carries food from the mouth to the stomach. The stomach further breaks down food through mechanical and chemical means, preparing it for absorption in the intestines. The liver plays a vital role in digestion as well by producing bile, which is essential for the emulsification and digestion of fats. In contrast, the kidney's main functions involve filtering blood to remove waste products and excess substances, thereby maintaining fluid and electrolyte balance in the body rather than being involved in the digestion of food.