**Core Concept**
The underlying condition is likely to be related to a metabolic disorder affecting calcium metabolism, resulting in an increased excretion of calcium in the urine. This is often associated with an increased risk of developing calcium-containing kidney stones.

**Why the Correct Answer is Right**
Hypercalciuria is a common cause of kidney stones, and it can be due to various factors, including increased absorption of calcium from the gut, increased bone resorption, or decreased renal excretion. The most common cause of hypercalciuria is primary hyperparathyroidism, where an overactive parathyroid gland leads to increased parathyroid hormone (PTH) levels, causing increased calcium reabsorption in the kidneys. However, in the context of haematuria, a more likely cause of hypercalciuria is a kidney stone, specifically a calcium oxalate stone, which can cause bleeding and damage to the renal epithelium.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the scenario described, as it does not address the relationship between haematuria and hypercalciuria.

**Option B:** Although hyperkalemia can cause kidney damage and potentially lead to haematuria, it is not directly related to hypercalciuria.

**Option C:** This option is not relevant to the scenario described, as it does not address the relationship between haematuria and hypercalciuria.

**Clinical Pearl / High-Yield Fact**
A common mnemonic to remember the causes of hypercalciuria is "THIRD": Thiazide diuretics, Hyperparathyroidism, Increased bone resorption, and Renal tubular acidosis.

**Correct Answer:** C. Calcium oxalate crystals