Which of the following kidney stones is often described as having a shape resembling an envelope?

The shape of calcium oxalate stones is often likened to an envelope, which is primarily due to the characteristic appearance of calcium oxalate dihydrate crystals. These crystals develop in a specific morphology that is rhomboid or octahedral, where the octahedral form can resemble an "envelope" when viewed under a microscope.

Calcium oxalate stones are the most common type of kidney stones, and they form when there is an excess of calcium and oxalate in the urine, leading to crystallization. The unique envelope-like appearance aids in their identification during microscopic examinations.

In contrast, uric acid stones typically present as more amorphous and may assume various shapes but do not have a distinctive envelope-like form. Struvite stones often appear in a more irregular and staghorn configuration due to their association with urinary tract infections, and cystine stones are hexagonal rather than resembling an envelope. Thus, recognizing the shape of calcium oxalate stones is crucial for understanding their formation and thereby their management.