Rectangular prism crystals seen in urine are which type?

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

Answer: (B)

Crystal shape in urine reflects its mineral composition, and the rectangular prism appearance is a classic description for struvite crystals, which are magnesium ammonium phosphate. These crystals often look like coffin lids or rectangular prisms, especially in neutral to alkaline urine and are commonly seen with UTIs caused by urease-producing bacteria. Calcium oxalate crystals are typically envelope-shaped or dumbbell-like, urate crystals are rhomboid or rosette-shaped, and calcium carbonate crystals have different simple forms, so the rectangular prism morphology fits struvite best. Therefore, when you observe rectangular prism crystals in urine, struvite is the most likely type.