American Board of Internal Medicine (ABIM) Certification Practice Exam

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What is the shape of calcium oxalate stones in nephrolithiasis?

Hexagonal
Rhomboid
Envelope/Dumbbell
Coffin-lid

The correct answer is: Envelope/Dumbbell

Calcium oxalate stones in nephrolithiasis are typically described as having an envelope or dumbbell shape. This characteristic appearance is due to the way calcium oxalate crystals form in urine, often leading to crystals that double in size and can join together to form larger stones. The envelope shape resembles two pyramids base to base, while the dumbbell shape emphasizes a more rounded aspect, making them easily identifiable under microscopic examination. This specific morphology is an important point for clinicians, as it helps differentiate calcium oxalate stones from other types of urolithiasis, such as struvite stones, which appear as coffin-lid shapes, or cystine stones, which are hexagonal. Recognizing the shape of these stones can assist in both diagnosis and treatment strategies for nephrolithiasis.