**Core Concept**
Squamous cell anal cancer is a type of cancer that arises from the squamous cells lining the anal canal. The treatment of choice for this condition involves a multidisciplinary approach, combining surgery, radiation therapy, and chemotherapy.

**Why the Correct Answer is Right**
The treatment of choice for squamous cell anal cancer is chemoradiation, which involves the concurrent administration of chemotherapy and radiation therapy. This approach is based on the principle that radiation therapy alone may not be sufficient to control the tumor, while chemotherapy can enhance the effectiveness of radiation by increasing the sensitivity of cancer cells to radiation-induced damage. The combination of chemotherapy and radiation therapy has been shown to improve local control and overall survival in patients with squamous cell anal cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** Surgery alone is not the treatment of choice for squamous cell anal cancer, as it may not be sufficient to control the tumor, especially in cases where the cancer is extensive or has spread to lymph nodes.
**Option B:** Radiation therapy alone may not be sufficient to control the tumor, as it may not be able to deliver a high enough dose of radiation to the tumor site.
**Option C:** Chemotherapy alone may not be sufficient to control the tumor, as it may not be able to deliver a high enough dose of chemotherapy to the tumor site.

**Clinical Pearl / High-Yield Fact**
The Nigro protocol, which involves the concurrent administration of chemotherapy and radiation therapy, is a well-established treatment regimen for squamous cell anal cancer.

**Correct Answer:** C. Chemoradiation.