**Core Concept**
Bladder stones, also known as vesical calculi, are small, hard mineral deposits that form inside the bladder. They can cause severe pain, urinary retention, and other complications if left untreated. The formation of bladder stones is a multifactorial process involving an imbalance of minerals, pH, and urine concentration.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of bladder stone formation. Bladder stones often occur in individuals with underlying conditions such as urinary tract infections, spinal cord injuries, or bladder outlet obstruction. The presence of an abnormal bacterial flora, particularly Proteus species, can contribute to the formation of bladder stones by promoting the growth of urease-producing bacteria. These bacteria break down urea into ammonia and carbon dioxide, increasing the pH of the urine and creating an environment conducive to stone formation.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because bladder stones are not typically associated with an excess of oxalate ions in the urine. While oxalate stones are a common type of kidney stone, they are not the primary type of stone found in the bladder.
* **Option B:** This option is incorrect because bladder stones are not usually caused by a deficiency of citrate in the urine. Citrate is a natural inhibitor of stone formation, and its deficiency is more commonly associated with kidney stone formation.
* **Option C:** This option is incorrect because bladder stones are not typically associated with an excess of calcium in the urine. While hypercalciuria is a risk factor for kidney stone formation, it is not a primary risk factor for bladder stone formation.

**Clinical Pearl / High-Yield Fact**
The most common type of bladder stone is the struvite stone, which is composed of magnesium ammonium phosphate. These stones are often associated with urinary tract infections caused by urease-producing bacteria.

**Correct Answer: D.**