**Core Concept**
The question is testing the student's understanding of the prognostic investigations required in colorectal cancer, specifically in the context of a patient with rectal cancer. The underlying principle being tested is the ability to identify unnecessary investigations.

**Why the Correct Answer is Right**
In the case of a patient with rectal cancer, the prognosis is often determined by the tumor's depth of invasion, lymph node involvement, and distant metastasis. A colectomy specimen on histopathology has already established the presence of adenocarcinoma, making further investigations like CT scan (A) or MRI (B) redundant for determining prognosis. However, investigations like CEA (Carcinoembryonic Antigen) levels (C) and lymphovascular invasion assessment (D) are relevant for determining prognosis.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan is useful for staging rectal cancer, particularly for identifying distant metastasis, but it is not a prognostic investigation in the context of histopathologically confirmed adenocarcinoma.
**Option B:** MRI is also useful for staging rectal cancer, but like CT scan, it is not a prognostic investigation in the context of histopathologically confirmed adenocarcinoma.
**Option C:** CEA levels are a prognostic marker in colorectal cancer, and elevated levels are associated with poor prognosis.
**Option D:** Lymphovascular invasion assessment is a prognostic factor in colorectal cancer, and its presence is associated with increased risk of recurrence.

**Clinical Pearl / High-Yield Fact**
In colorectal cancer, the prognosis is often determined by the tumor's depth of invasion, lymph node involvement, and distant metastasis. The presence of lymphovascular invasion and elevated CEA levels are also important prognostic factors.

**Correct Answer:** D. Lymphovascular invasion assessment is a prognostic factor in colorectal cancer, and its presence is associated with increased risk of recurrence.