**Core Concept**
The assessment of depth of penetration and perirectal nodes in rectal cancer is crucial for staging and planning treatment. The investigation of choice should provide detailed imaging of the rectal wall and surrounding tissues to accurately determine the extent of tumor spread.

**Why the Correct Answer is Right**
Magnetic Resonance Imaging (MRI) is the investigation of choice for assessing the depth of penetration and perirectal nodes in rectal cancer. MRI provides high-resolution images of the rectal wall and surrounding tissues, allowing for accurate staging of the tumor. It is particularly useful for evaluating the mesorectal fascia, which is the key factor in determining the depth of tumor penetration. MRI also helps in identifying perirectal lymph nodes, which are critical for staging and planning treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan is not the investigation of choice for assessing the depth of penetration and perirectal nodes in rectal cancer. While CT scan can provide some information on the extent of tumor spread, it is not as sensitive as MRI for evaluating the rectal wall and surrounding tissues.

**Option B:** Endoscopic ultrasound (EUS) is useful for evaluating the depth of tumor penetration within the rectal wall but is not ideal for assessing perirectal nodes.

**Option C:** PET scan is not the investigation of choice for assessing the depth of penetration and perirectal nodes in rectal cancer. While PET scan can provide information on tumor metabolism and spread, it is not as useful as MRI for evaluating the extent of tumor spread in the rectum and surrounding tissues.

**Clinical Pearl / High-Yield Fact**
MRI should be performed with a phased-array coil to improve image quality and accuracy.

**Correct Answer: C. PET scan is not the investigation of choice for assessing the depth of penetration and perirectal nodes in rectal cancer. While PET scan can provide information on tumor metabolism and spread, it is not as useful as MRI for evaluating the extent of tumor spread in the rectum and surrounding tissues.