**Core Concept:** Colonoscopy is the gold standard investigation for lower gastrointestinal bleeding. Various treatment options are available based on the findings during colonoscopy, including polypectomy, endoscopic mucosal resection, thermal/chemical coagulation, and surgery.

**Why the Correct Answer is Right:** In this case, the patient presents with symptoms and signs of lower gastrointestinal bleeding, and sigmoidoscopy reveals a mass measuring 4 cm about 3.5 cm above the anal verge. Colonoscopy is the appropriate next step due to its superior diagnostic capabilities over sigmoidoscopy. The mass found during sigmoidoscopy indicates the presence of a polyp or malignancy, which can be treated directly during colonoscopy.

**Why Each Wrong Option is Incorrect:**

A. Biopsy: While biopsy is an essential component of colonoscopy, it is not a treatment option in this case.

B. Sigmoidoscopy: Sigmoidoscopy is a limited procedure and not suitable for treating the identified mass due to its restricted reach.

C. Endoscopy: Endoscopy is a general term for gastrointestinal endoscopy, but it is too broad and not specific enough to address the patient's condition accurately.

D. Surgery: Surgery is a treatment option for certain colorectal lesions, but it is not the best choice for treating the identified mass during colonoscopy due to its invasive nature and potential complications.

**Clinical Pearl:** Colonoscopy is essential in diagnosing and treating lower gastrointestinal lesions, ensuring accurate diagnosis and appropriate management while minimizing the risk of complications.

**Correct Answer:** Colonoscopy.