Okay, let's tackle this question. The patient is a 31-year-old woman with a history of three UTIs in the past year. She presents with abdominal pain, hematuria, and blood in her urine. Urinalysis shows 2+ hematuria, 1+ proteinuria, hypercalciuria, and no glucose or ketones. Serum creatinine is normal. Microscopic examination reveals RBCs and oxalate crystals. The CT scan shows linear striations in the renal papillae and small cystic collections in dilated collecting ducts. The treatment was increased fluids and her condition improved.

First, I need to figure out which renal cystic disease is associated with these findings. The key points here are the presence of oxalate crystals, hypercalciuria, and the CT findings. The CT scan's linear striations and cystic collections in the collecting ducts are classic for a certain condition.

Renal cystic diseases include conditions like autosomal dominant polycystic kidney disease (ADPKD), medullary sponge kidney, and others. ADPKD typically presents with larger cysts and is often associated with hypertension, hematuria, and a family history. However, the CT findings here are more specific. Medullary sponge kidney is characterized by cystic dilatation of the collecting ducts in the renal medulla, which can lead to hypercalciuria, hematuria, and UTIs. The oxalate crystals might be due to hypercalciuria leading to calcium oxalate deposition.

The presence of linear striations on CT is a hallmark of medullary sponge kidney. The small cystic collections in the collecting ducts would correspond to the dilated ducts seen in this condition. The improvement with increased fluid intake supports the diagnosis because hydration helps prevent stone formation, which is common in this disease.

Other options might include conditions like ADPKD, but ADPKD usually has larger cysts and different imaging findings. Another possibility could be nephrolithiasis, but the question asks for a renal cystic disease. Nephrocalcinosis could be considered, but the CT findings here are more specific for medullary sponge kidney.

So, the correct answer is likely medullary sponge kidney. The key features are the CT findings, hypercalciuria, and recurrent UTIs. The treatment with increased fluids is aimed at preventing stone formation, which aligns with the management of medullary sponge kidney.

**Core Concept**
This question tests understanding of **medullary sponge kidney (MSK)**, a rare congenital renal cystic disease characterized by **cystic dilatation of the collecting ducts** in the renal medulla. It is associated with **hypercalciuria**, **calcium oxalate nephrolithiasis**, and recurrent **urinary tract infections (UTIs)** due to stasis of urine in the dilated ducts.

**Why the Correct Answer is Right**
Medullary sponge kidney (MSK) is caused by **abnormal development of the collecting ducts**, leading to **cystic dilatation** and **calcium oxalate deposition**. The **linear striations** on CT represent **calcified papillary cysts**, while **small cystic collections** correspond to dilated collecting ducts. Hypercalciuria

✓ Correct Answer: C. Medullary sponge kidney