**Core Concept**
The question tests the student's ability to identify a specific type of kidney stone based on clinical presentation and laboratory findings. The presence of hexagonal crystals and a positive cyanide-nitroprusside test in the urine are key diagnostic clues.

**Why the Correct Answer is Right**
The hexagonal crystals observed in the urinalysis are characteristic of uric acid stones. The cyanide-nitroprusside test is a specific test for detecting uric acid in the urine. This test reacts with uric acid to produce a purple color, indicating a positive result. Uric acid stones are often associated with conditions such as gout, high levels of uric acid in the blood, or a diet rich in meat and seafood. In this case, the spontaneous passage of the stone and the presence of hexagonal crystals and a positive cyanide-nitroprusside test support the diagnosis of uric acid stones.

**Why Each Wrong Option is Incorrect**
* **Option A:** Calcium oxalate stones are the most common type of kidney stone, but they typically do not produce a positive cyanide-nitroprusside test. They may be associated with factors such as a high-oxalate diet, certain medical conditions, or family history.
* **Option B:** Cystine stones are rare and typically occur in patients with cystinuria, a genetic disorder that affects the kidneys' ability to reabsorb cystine. They are not associated with a positive cyanide-nitroprusside test.
* **Option D:** Struvite stones are often associated with urinary tract infections (UTIs) and are typically seen in women. They are not typically identified by a positive cyanide-nitroprusside test.

**Clinical Pearl / High-Yield Fact**
Uric acid stones are more likely to occur in individuals with high levels of uric acid in the blood, such as those with gout or those consuming a diet rich in meat and seafood. The cyanide-nitroprusside test is a specific and sensitive test for detecting uric acid in the urine.

**Correct Answer:** C. Uric acid stones.