**Core Concept**
Secondary bladder stones are a type of urinary stone that forms in the bladder, distinct from primary stones that originate in the kidneys. The formation of secondary bladder stones is often related to the presence of an underlying condition or a specific set of circumstances that alter the normal urinary environment.

**Why the Correct Answer is Right**
Secondary bladder stones typically form due to the presence of a foreign body or a nidus within the bladder, which acts as a site for crystal formation and aggregation. This can be caused by a retained Foley catheter, a ureteral stent, or even a tumor. The altered urine environment, often with increased pH, concentration, and presence of certain substances, facilitates the precipitation of minerals and the growth of a stone. In this context, the correct answer is related to the presence of a foreign body or a nidus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because primary bladder stones are a distinct entity and are not formed due to the same mechanisms as secondary stones.
**Option B:** This option is incorrect because urinary tract infections (UTIs) can contribute to the formation of primary stones, but they are not a direct cause of secondary bladder stones.
**Option C:** This option is incorrect because the presence of an underlying metabolic disorder can contribute to the formation of primary stones, but it is not a specific cause of secondary bladder stones.

**Clinical Pearl / High-Yield Fact**
When managing patients with secondary bladder stones, it is essential to identify and address the underlying cause or foreign body that facilitated stone formation, in addition to standard stone treatment protocols.

**Correct Answer:** C.