Stone which is resistant to lithotripsy:
**Core Concept**
Lithotripsy is a non-invasive treatment for kidney stones that uses shockwaves to break them into smaller fragments that can then pass out of the body. The effectiveness of lithotripsy depends on the composition and location of the stone.
**Why the Correct Answer is Right**
The correct answer is a struvite stone (magnesium ammonium phosphate). Struvite stones are formed from the combination of magnesium, ammonium, and phosphate ions, which can occur in patients with urinary tract infections (UTIs) caused by certain bacteria, such as Proteus or Pseudomonas. These stones are particularly resistant to lithotripsy due to their complex crystal structure and the presence of magnesium, which can absorb the shockwaves and prevent fragmentation.
**Why Each Wrong Option is Incorrect**
* **Option A:** Calcium oxalate stones are the most common type of kidney stone and are often responsive to lithotripsy. However, they can be large and may require multiple sessions to achieve complete fragmentation.
* **Option B:** Uric acid stones are typically softer than other types of stones and can be easily broken up with lithotripsy. They are often associated with low urine pH and may be seen in patients with gout or those taking certain medications.
* **Option C:** Cystine stones are rare and occur in patients with cystinuria, a genetic disorder that causes the kidneys to reabsorb cystine from the urine. While they can be resistant to lithotripsy, they are not as resistant as struvite stones.
**Clinical Pearl / High-Yield Fact**
Struvite stones are often associated with urinary tract infections (UTIs) and can be prevented with the use of antibiotics in patients with recurrent UTIs.
**Correct Answer:** D. Struvite stone (magnesium ammonium phosphate).