**Core Concept**
Medullary sponge kidney disease is a congenital disorder characterized by cystic dilation of the collecting ducts in the kidneys, leading to impaired renal concentration of urine and an increased risk of urinary tract infections. This condition results from abnormal development of the renal collecting system.

**Why the Correct Answer is Right**
The correct answer is related to the clinical manifestations of medullary sponge kidney disease. Patients with this condition often present with hematuria, flank pain, and recurrent urinary tract infections due to the abnormal development of the renal collecting system. The cystic dilation of the collecting ducts can also lead to impaired renal concentration of urine, resulting in polyuria or nocturia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while medullary sponge kidney disease can cause hematuria, it is not the most specific symptom associated with this condition.
**Option B:** This option is incorrect because flank pain is a symptom of medullary sponge kidney disease, but it is not as specific or pathognomonic as hematuria.
**Option C:** This option is incorrect because while medullary sponge kidney disease can increase the risk of kidney stones, it is not the most direct symptom of the condition.
**Option D:** This option is incorrect because nephrotic syndrome is not a symptom of medullary sponge kidney disease.

**Clinical Pearl / High-Yield Fact**
A key feature of medullary sponge kidney disease is the presence of cystic dilation of the collecting ducts, which can lead to impaired renal concentration of urine and an increased risk of urinary tract infections.

**Correct Answer: A. Hematuria**