**Core Concept**
Urinary retention in a 70-year-old man, combined with back pain, suggests a possible urinary tract obstruction, likely caused by an enlarged prostate or a mass effect on the urinary tract. The clinical presentation points towards a need for imaging to identify the cause of obstruction.

**Why the Correct Answer is Right**
The correct answer involves performing an ultrasound to evaluate the prostate size and any potential masses or obstructions in the urinary tract. This is because ultrasound is a quick, non-invasive, and cost-effective imaging modality that can provide valuable information about the prostate and urinary tract. Additionally, it does not involve ionizing radiation, making it suitable for older patients. The ultrasound can help identify an enlarged prostate, bladder stones, or a mass effect that could be causing the urinary retention.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan - While a CT scan can provide detailed images of the urinary tract, it involves ionizing radiation and is more expensive than an ultrasound. In a 70-year-old patient, the risk of radiation exposure outweighs the benefits.

**Option B:** MRI - MRI is not typically used as a first-line imaging modality for urinary retention due to its higher cost and limited availability compared to ultrasound.

**Option C:** IVP - Intravenous pyelogram (IVP) is an older imaging technique that involves injecting contrast into the bloodstream to visualize the urinary tract. It is less commonly used today due to the availability of ultrasound and CT scans.

**Clinical Pearl / High-Yield Fact**
In older men with urinary retention and back pain, an enlarged prostate is a common cause of obstruction. A digital rectal examination (DRE) can help identify an enlarged prostate, but imaging is often necessary to rule out other causes.

**Correct Answer:** C. Ultrasound of the prostate and urinary tract.