American Board of Internal Medicine (ABIM) Certification Practice Exam

What is the characteristic appearance of struvite stones in nephrolithiasis?

1. Coffin shape

2. Hexagonal shape

3. Rhomboid shape

4. Star shape

The correct answer is: Coffin shape

Struvite stones, which are associated with urinary tract infections, particularly those caused by urease-producing bacteria, display a characteristic appearance that resembles a coffin. This distinctive coffin shape results from the crystallization of magnesium ammonium phosphate and is often noted during imaging studies or examination of the stones themselves. The formation of struvite stones is typically linked to alkaline urine, which promotes their development, and they can grow quite large, sometimes leading to the formation of "staghorn" calculi that fill the renal pelvis and calyces. The other shapes mentioned, such as hexagonal, rhomboid, or star shapes, are associated with different types of stones. For example, hexagonal stones are characteristic of cystine stones, while rhomboid shapes are often linked to uric acid stones. Each type of stone has unique biochemical pathways and precipitating conditions that lead to their formation, enabling differentiation based on their appearance and associated clinical conditions. Understanding these characteristics is crucial for accurate diagnosis and management in nephrolithiasis.