**Core Concept**
Flexible sigmoidoscopy is a diagnostic procedure that allows for visualization of the sigmoid colon and rectum using a flexible endoscope. The length of the flexible sigmoidoscope is crucial for determining the extent of the procedure and the potential risks associated with it.

**Why the Correct Answer is Right**
The standard length of a flexible sigmoidoscope is typically between 60-70 cm, allowing for examination of the sigmoid colon and rectum. This length allows for adequate visualization and sampling of the colonic mucosa while minimizing the risk of complications. The flexible sigmoidoscope is designed to be inserted through the anus and advanced into the sigmoid colon, where it can be used to collect tissue samples or perform therapeutic interventions.

**Why Each Wrong Option is Incorrect**
**Option A:** 30 cm - This length is too short for a flexible sigmoidoscope, as it would not allow for adequate examination of the sigmoid colon and rectum. A scope of this length would be more suitable for a pediatric or small animal examination.
**Option B:** 100 cm - This length is too long for a flexible sigmoidoscope, as it would likely exceed the reach of the sigmoid colon and pose an increased risk of complications, such as bowel perforation.
**Option C:** 20 cm - This length is extremely short and would not allow for any examination of the sigmoid colon or rectum.

**Clinical Pearl / High-Yield Fact**
When selecting a flexible sigmoidoscope, it is essential to choose a scope that is the correct length for the patient's anatomy, as using a scope that is too long or too short can increase the risk of complications.

**Correct Answer:** 65 cm **Correct Answer:** 65 cm