Question

Which kidney stone is radiolucent on a plain KUB X-ray?

• A. \(Calcium\ oxalate\ stone\)
• B. \(Struvite\ stone\)
• C. \(Uric\ acid\ stone\)
• D. \(Calcium\ phosphate\ stone\)

Hint:

The "I Can't See U" mnemonic: "I" for invisible, "U" for Uric acid. Most stones are radiopaque; Uric acid, Crixivan (Indinavir), and Xanthine stones are the primary radiolucent ones.

Answer 1

The Correct Option is C

Concept:

• Radiopacity of a kidney stone depends on its mineral composition and density relative to surrounding tissues.

• Stones containing heavy atoms like Calcium (atomic number 20) are radiopaque, meaning they appear white on an X-ray.

• Stones made of light elements (Carbon, Hydrogen, Nitrogen, Oxygen) are radiolucent and do not show up on plain X-ray films.
Step 1: Analyzing radiopaque stones
Calcium oxalate and Calcium phosphate stones are the most common types and are highly radiopaque due to the high density of calcium.
Struvite stones (Magnesium Ammonium Phosphate) are also radiopaque, though often less dense than pure calcium stones.

Step 2: Analyzing radiolucent stones
Uric acid stones are composed of uric acid crystals, which consist of light organic elements.
Because these elements do not significantly attenuate X-ray beams, the stones are "invisible" or radiolucent on a plain Kidney-Ureter-Bladder (KUB) X-ray.

Step 3: Clinical Diagnosis
While they cannot be seen on a plain X-ray, uric acid stones can be visualized using other imaging modalities such as:

• Non-contrast CT scan (most accurate).

• Ultrasound (shows as a hyperechoic focus with posterior acoustic shadowing).

• Intravenous Pyelogram (shows as a filling defect).