Final answer:
The correct answer is Ureters. Extreme back pain occurs when sand-like calculi, also known as kidney stones or renal calculi, pass from the kidneys to the bladder through the ureters.
Step-by-step explanation:
The correct answer is a) Ureters.
Extreme back pain occurs when sand-like calculi, also known as kidney stones or renal calculi, pass from the kidneys to the bladder through the ureters. The ureters are slender tubes that connect the kidneys to the bladder. As the kidney stones travel through the ureters, they can cause obstruction and severe pain in the back, lower abdomen, and groin (renal colic).