Which of the following structures is not an organ of the disgestive system?

Analyze the question: You're asked to find the structure that does NOT belong to the digestive system. Three options are digestive organs and one is not.

Explain the concept: The digestive system breaks down food into small, absorbable molecules. It includes the alimentary canal (mouth, pharynx, oesophagus, stomach, small intestine, large intestine, rectum, anus) and accessory organs (salivary glands, liver, gall bladder, pancreas) that secrete digestive juices.

Go through each option:

• A. Oesophagus: The food pipe. It moves food from the mouth to the stomach by peristalsis. It is part of the digestive system.
• B. Pancreas: An accessory digestive organ. It secretes pancreatic juice containing amylase, trypsin and lipase into the duodenum. It is part of the digestive system. (It also makes insulin, but it still counts as digestive.)
• C. Stomach: Churns food and secretes gastric juice (HCl and pepsin) for protein digestion. It is part of the digestive system.
• D. Kidney: Filters blood and removes urea and excess water and salts as urine. This is excretion/osmoregulation, so it belongs to the excretory (urinary) system, not the digestive system.

Exam tip: Group organs by system when you revise:

• Digestive: oesophagus, stomach, intestines, liver, pancreas
• Excretory: kidney, ureter, bladder, skin, lungs

When a question asks for the "odd one out," ask yourself, "What is each organ's main job?" The kidney's job is excretion, not digestion.