**Core Concept**
The investigation of choice for assessing depth of penetration and perirectal node involvement in rectal cancer involves visualizing the tumor's relationship with surrounding tissues and lymph nodes. This requires high-resolution imaging of the rectal wall and surrounding pelvic structures.

**Why the Correct Answer is Right**
MRI (Magnetic Resonance Imaging) scan is the preferred method for assessing depth of penetration and perirectal node involvement in rectal cancer due to its superior soft-tissue resolution. It allows for accurate evaluation of the T-stage (depth of tumor invasion) and N-stage (lymph node involvement) of the cancer. MRI can clearly differentiate between different layers of the rectal wall and identify involved lymph nodes in the perirectal space. This information is crucial for determining the optimal treatment plan, including neoadjuvant therapy and surgical approach.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan pelvis, while useful for staging rectal cancer, has limited resolution for assessing depth of penetration and perirectal node involvement compared to MRI.
**Option B:** Double contrast barium enema is primarily used for evaluating the lumen of the rectum and is not suitable for assessing depth of penetration or perirectal node involvement.
**Option C:** Transrectal ultrasound (TRUS) is useful for local staging of rectal cancer but has limited ability to evaluate perirectal lymph nodes and is not as accurate as MRI for assessing depth of penetration.

**Clinical Pearl / High-Yield Fact**
When assessing rectal cancer, it's essential to remember that MRI is superior to CT scan for evaluating the T-stage and N-stage, and should be used as the primary imaging modality for preoperative staging.

**✓ Correct Answer: A. MRI scan**