Which crystal type is described as a rectangular prism?

• A. Struvite crystals (triple magnesium phosphate)
• B. Calcium carbonate crystals
• C. Calcium oxalate crystals
• D. Urate crystals

Answer: (A)

Crystal morphology in urine helps differentiate crystal types. Struvite crystals, made of magnesium ammonium phosphate, commonly appear as rectangular prisms with flat faces and straight edges, often described as coffin-lid shapes. This rectangular prism form is a defining feature used to recognize struvite under urine sediment examination, especially when the urine is alkaline and UTIs with urease-producing bacteria are involved, which promotes struvite formation. In contrast, calcium oxalate crystals are typically envelope- or dumbbell-shaped, urate crystals are needle-like or rosette-shaped, and calcium carbonate crystals often look blocky or dumbbell-like but not as a rectangular prism. So the rectangular prism appearance points to struvite crystals.