**Core Concept**
The patient's presentation of bright red blood coating the outside of the stools, pencil-thin stools, and a change in caliber of the stools over several weeks is suggestive of rectal bleeding with a possible obstruction. This clinical picture is often associated with rectal cancer, which is a type of colorectal cancer that arises from the mucosa of the rectum.

**Why the Correct Answer is Right**
The patient's symptoms are consistent with rectal cancer, particularly the presence of bright red blood and pencil-thin stools, which indicate a partial obstruction of the rectum. Rectal cancer typically arises from the mucosa of the rectum and can cause bleeding, changes in bowel habits, and a decrease in the caliber of the stools. The rectum is a common site for colorectal cancer, and the patient's age and symptoms make rectal cancer a likely diagnosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Anal fissure typically presents with pain during defecation, which is not consistent with the patient's symptom-free status. Additionally, anal fissures are usually associated with small amounts of bright red blood on the surface of the stool, but not the coating of the outside of the stool seen in this patient.
* **Option B:** Cancer of the cecum is a type of colorectal cancer that arises from the cecum, the first part of the large intestine. While it can cause changes in bowel habits and bleeding, it is less likely to cause pencil-thin stools and bright red blood coating the outside of the stool.
* **Option D:** External hemorrhoids typically present with pain, itching, and bright red blood on the surface of the stool, but not the coating of the outside of the stool seen in this patient.

**Clinical Pearl / High-Yield Fact**
A "double contrast barium enema" is a radiographic examination that can help diagnose rectal cancer by showing the tumor as a filling defect within the rectum. This test can also help identify the location and extent of the tumor, which is critical for surgical planning.

**✓ Correct Answer: C. Cancer of the rectum**