**Core Concept**
The patient presents with obstructive uropathy, characterized by a poor stream of urine, elevated post-void residual urine volume, and hydronephrosis, likely due to bladder outlet obstruction. The prostate is enlarged, weighing 70g, which is a strong indicator of benign prostatic hyperplasia (BPH) or prostatic hypertrophy.

**Why the Correct Answer is Right**
The immediate goal is to relieve the obstruction and prevent further renal damage. Catheterization with a Foley catheter is the most appropriate next step to directly address the obstruction and monitor urinary output. This procedure allows for the measurement of urinary output, assessment of bladder function, and potential removal of the catheter if the obstruction is relieved. The patient's elevated urea and creatinine levels indicate impaired renal function, necessitating prompt intervention.

**Why Each Wrong Option is Incorrect**
**Option A:** Performing CT or MRI pelvis would be a diagnostic step, but it would not address the immediate need to relieve the obstruction. These imaging modalities may take time to arrange and perform, delaying potential treatment.
**Option B:** BA PC Nephrostomies are a more invasive procedure typically reserved for patients with severe obstructive uropathy or those who are not candidates for catheterization. It is not the next immediate step in this scenario.
**Option C:** Ruling out cancer with CT is essential, but it should not take precedence over relieving the obstruction. A Foley catheter can be placed while awaiting imaging results.

**Clinical Pearl / High-Yield Fact**
In patients with suspected obstructive uropathy, a Foley catheter should be placed to monitor urinary output and assess bladder function before proceeding with further diagnostic or therapeutic interventions.

**✓ Correct Answer: A. Catheterize with Foley catheter**