What is the primary characteristic of a fistula?

A fistula is primarily characterized by being an abnormal passage that connects two organs or vessels that normally do not have a connection. This can occur due to various medical conditions, trauma, or surgical complications, leading to unintended pathways through which fluids or other substances can move. For example, a common type of fistula may occur between the intestines and the bladder, allowing urine to exit through the intestines.

The options that do not describe a fistula involve different medical conditions or structures. Involuntary muscle contraction pertains to muscle spasms or seizures, inflammation of skin tissue refers to dermatological issues like rashes or infections, and malformation of blood vessels involves vascular anomalies like aneurysms or varicosities. These conditions are distinct and do not relate to the specific definition and implications of a fistula in the medical context.