**Question:** Which of the following stones is hard to break by ESWL?

A. Kidney stone made of calcium oxalate monohydrate (COM)
B. Kidney stone made of cystine
C. Kidney stone made of uric acid
D. Kidney stone made of calcium phosphate

**Core Concept:** Extracorporeal Shock Wave Lithotripsy (ESWL) is a minimally invasive treatment for kidney stones. ESWL generates high-intensity shock waves that break the kidney stones into smaller fragments, which can then be passed through the urinary tract. This treatment is effective for most types of kidney stones, but some stones are more resistant to ESWL due to their composition.

**Why the Correct Answer is Right:** Kidney stone made of calcium phosphate (option D) is harder to break by ESWL compared to the other options. Calcium phosphate stones are more resistant to shock waves because they have a higher crystalline structure, which reduces the penetration and effectiveness of the shock waves.

**Why Each Wrong Option is Incorrect:**

A. Kidney stone made of calcium oxalate monohydrate (COM) is easier to break by ESWL (option A). Calcium oxalate monohydrate stones have a lower crystalline structure, allowing shock waves to penetrate more effectively and break the stones into smaller fragments.

B. Kidney stone made of cystine (option B) is also relatively easy to break by ESWL. Cystine stones are formed from a single amino acid and have a lower crystalline structure, making them more susceptible to ESWL treatment.

C. Kidney stone made of uric acid (option C) is usually easier to treat with ESWL. Uric acid stones have a lower crystalline structure, allowing shock waves to penetrate and break the stones effectively.

**Clinical Pearl:** ESWL treatment success depends on the stone composition. It is crucial to accurately identify the stone type before choosing the most appropriate treatment method. In some cases, ESWL might not be effective, and alternative treatments like ureteroscopy or percutaneous nephrolithotomy may be needed.