**Core Concept**
The patient's symptoms of altered bowel habit and rectal bleeding in a 61-year-old individual warrant an evaluation for colorectal cancer, a common malignancy in this age group. The initial approach involves a non-invasive, cost-effective screening tool to assess for colorectal neoplasia.

**Why the Correct Answer is Right**
The next best investigation is a fecal occult blood test (FOBT), which detects hidden blood in the stool. This test is particularly useful for individuals who refuse colonoscopy. FOBT is a simple, non-invasive test that can identify occult gastrointestinal bleeding, which is often associated with colorectal cancer. The test involves collecting stool samples at home and sending them to a laboratory for analysis. If the test is positive, further evaluation with colonoscopy or other imaging modalities may be considered.

**Why Each Wrong Option is Incorrect**
* **Option A:** Imaging modalities such as CT colonography (virtual colonoscopy) may be used as an alternative to colonoscopy in certain cases, but they are not the next best investigation in this scenario, as they are more expensive and may not be readily available.
* **Option B:** Flexible sigmoidoscopy is a procedure that examines the lower part of the colon, but it is not the next best investigation in this scenario, as the patient's symptoms suggest a more proximal lesion.
* **Option D:** MRI colonography is a newer imaging modality that can visualize the colon, but it is not commonly used for this purpose and is not the next best investigation in this scenario.

**Clinical Pearl / High-Yield Fact**
In patients who refuse colonoscopy, a fecal occult blood test (FOBT) is a simple, non-invasive screening tool that can identify occult gastrointestinal bleeding and guide further evaluation.

**Correct Answer:** C.