American Board of Internal Medicine (ABIM) Certification Practice Exam

Which of the following stones has a wedge/prism shape?

1. Calcium Oxalate

2. Uric Acid

3. Calcium Phosphate

4. Ammonium Magnesium Phosphate

The correct answer is: Calcium Phosphate

Calcium phosphate stones are characteristically known for their wedge or prism shapes. This distinct morphology is primarily due to the crystalline structure of calcium phosphate, which tends to form in a more elongated, geometric shape as it precipitates in the renal tubules. These stones are often associated with conditions that lead to alkaline urine and can be seen in patients with renal tubular acidosis or certain infections. Their unique wedge-shaped or prism-like appearance can be observed under polarized light microscopy, distinguishing them from other types of urinary stones. Other types of stones, such as calcium oxalate, uric acid, or ammonium magnesium phosphate, exhibit different shapes and characteristics. For instance, calcium oxalate stones can appear as envelope-shaped (found in the monohydrate form) or dumbbell-shaped (found in the dihydrate form), while uric acid stones are typically smooth and less crystalline. Ammonium magnesium phosphate stones often take the form of struvite, which can resemble coffin lids or have a more irregular shape associated with infection-mediated precipitation. Understanding the morphology of urinary stones is important for diagnosis and management, as it can provide insights into underlying metabolic conditions and guide appropriate treatment strategies.