## Core Concept
Jack stone calculi, also known as jack-shaped stones, are a type of urinary calculus. These stones are typically composed of a specific type of material that gives them their characteristic shape. The composition of urinary calculi can vary, including calcium oxalate, uric acid, struvite, and cystine stones.

## Why the Correct Answer is Right
The correct answer, **C. Calcium oxalate**, is right because jack stone calculi are characteristically made of calcium oxalate. Calcium oxalate stones are one of the most common types of kidney stones and can form in acidic urine. The unique shape of jack stone calculi, resembling a jack or a spiky ball, is often associated with calcium oxalate stones due to their crystalline structure.

## Why Each Wrong Option is Incorrect
- **Option A:** Uric acid stones are typically small, smooth, and radiolucent, not resembling the jack stone configuration.
- **Option B:** Struvite stones, also known as magnesium ammonium phosphate stones, are usually associated with urinary tract infections (UTIs) and have a different appearance.
- **Option D:** Cystine stones are rare and occur in patients with cystinuria; they are typically smooth and not characteristically jack-shaped.

## Clinical Pearl / High-Yield Fact
A memorable point for exams is that the presence of certain types of stones can indicate specific conditions or risk factors, such as diet, dehydration, or metabolic disorders. For calcium oxalate stones, prevention strategies include dietary modifications and increased fluid intake.

## Correct Answer: C. Calcium oxalate.