The circular ring of muscle that constricts a passage or closes a natural opening is called:

• A. Sphincter
• B. Valve
• C. Tendon
• D. Ligament

Answer: (A)

The concept being tested is that a circular ring of muscle around an opening that tightens to close the passage is called a sphincter. A sphincter is a ring-shaped muscle that contracts to constrict the diameter of a passage or seal an opening, regulating flow. These muscles can be smooth (involuntary) or skeletal (voluntary) depending on their location, with examples like the lower esophageal sphincter, pyloric sphincter, and external anal sphincter illustrating how they control movement and continence. The other terms describe different structures: a valve is a flap-like mechanism that regulates flow but isn’t a muscle; a tendon attaches muscle to bone; a ligament connects bone to bone.