**Core Concept**
The question is testing the knowledge of radiation therapy fields in cervical cancer. The pelvic lymph nodes are a common site for metastasis in cervical cancer, and radiation therapy is often used to treat these nodes. However, not all lymph nodes in the pelvis are treated equally due to differences in their anatomical location and potential impact on surrounding structures.

**Why the Correct Answer is Right**
The external radiation therapy field for cervical cancer typically includes the pelvic lymph nodes, specifically the obturator, internal iliac, and sacral nodes. However, the external iliac nodes are often excluded from the radiation field due to their proximity to the femoral heads and the potential for radiation-induced osteonecrosis. The external iliac nodes are also less likely to harbor cancer in early-stage cervical cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** The obturator nodes are included in the radiation field due to their involvement in the spread of cervical cancer.
**Option B:** The internal iliac nodes are also included in the radiation field as they are a common site for metastasis in cervical cancer.
**Option C:** The sacral nodes are included in the radiation field as they are part of the pelvic lymphatic drainage of the cervix.

**Clinical Pearl / High-Yield Fact**
Remember the "OIL" acronym to recall the lymph nodes that are typically included in the radiation field for cervical cancer: Obturator, Internal Iliac, and Sacral nodes.

**Correct Answer: D. External iliac nodes are often excluded from the radiation field due to their proximity to the femoral heads and the potential for radiation-induced osteonecrosis.**