Which of the following is a cathartic?

A cathartic is a substance that induces or stimulates bowel movements. It is commonly used to treat constipation and prepare patients for certain medical procedures, such as radiologic imaging or surgeries.

Castor oil is known for its strong laxative properties and is classified as a cathartic because it promotes intestinal peristalsis, leading to increased bowel movement. The active component in castor oil, ricinoleic acid, triggers this effect by irritating the lining of the intestine and increasing the secretion of fluids into the bowel.

In contrast, digitalis is primarily used to treat heart conditions, furosemide is a diuretic that promotes fluid excretion rather than bowel movement, and phenobarbital is a sedative medication used to control seizures and anxiety. Therefore, while these medications serve important roles in patient care, they do not have the properties or functions associated with a cathartic.