## Core Concept
The patient's presentation with intermittent constipation and microcytic anemia, along with a barium enema showing a non-obstructing "apple-core" lesion of the proximal sigmoid colon, is highly suggestive of a **malignant colorectal tumor**. The "apple-core" appearance is characteristic of a stenosing, annular carcinoma of the colon. Given the location and the absence of synchronous lesions, the next step involves planning for **surgical resection**, which is a primary treatment for localized colorectal cancer.

## Why the Correct Answer is Right
The correct approach involves **surgical resection** of the tumor, which is the primary treatment for localized colorectal cancer. This approach aims to remove the tumor completely, assess for lymph node involvement, and provide the best chance for a cure. The surgical approach might involve a **segmental colectomy**, which includes removing the portion of the colon containing the tumor along with its blood supply and associated lymph nodes.

## Why Each Wrong Option is Incorrect
- **Option A:** While **colonoscopy** has been performed to confirm the location and nature of the lesion, repeating it without any new indication (like a change in symptoms or a need for further evaluation) would not be the immediate next step.
- **Option B:** **Biopsy** of the lesion was likely performed during colonoscopy to confirm the diagnosis of cancer. However, once a diagnosis of cancer is made, the next step is not to repeat the biopsy but to plan for treatment.
- **Option C:** **Radiation therapy** might be considered in certain cases of rectal cancer or for palliation, but it is not the primary next step for a resectable sigmoid colon cancer.
- **Option D:** This option is not provided, but based on the context, any option not leading to surgical intervention or proper staging (like imaging) would be incorrect.

## Clinical Pearl / High-Yield Fact
A key point to remember is that for **resectable colorectal cancers**, surgical intervention is often the primary treatment that offers the best chance of cure. Additionally, the presence of microcytic anemia suggests chronic blood loss, which is a common presentation of colorectal cancer.

## Correct Answer Line
**Correct Answer: .**