## Why Internal iliac lymph nodes is right The region marked **A** (above the pectinate line) has visceral innervation and is supplied by the superior rectal artery (branch of the inferior mesenteric artery). Critically, lymphatic drainage from structures above the pectinate line flows to the **internal iliac lymph nodes**, not the superficial inguinal nodes. This is a cardinal anatomical principle: adenocarcinomas arising above the pectinate line metastasize first to internal iliac nodes. The columnar epithelium and painless bleeding are also characteristic of pathology above the pectinate line (Gray's Anatomy 42e, Bailey & Love 28e). ## Why each distractor is wrong - **Superficial inguinal lymph nodes**: These drain the region BELOW the pectinate line (somatic territory). Below-the-line pathology (external hemorrhoids, squamous cell carcinoma) metastasizes to inguinal nodes, not above-the-line lesions. - **Mesenteric lymph nodes**: While the superior rectal artery is a branch of the IMA, the primary lymphatic drainage from above the pectinate line is to internal iliac nodes, not superior mesenteric nodes. Mesenteric nodes are involved in more proximal colorectal cancers. - **Obturator lymph nodes**: These are not the primary drainage pathway for anal canal structures above the pectinate line and are not a standard answer for anal canal malignancy staging. **High-Yield:** Above pectinate line = internal iliac nodes; below pectinate line = superficial inguinal nodes. This distinction is critical for staging anal canal cancers and predicting metastatic spread. [cite: Gray's Anatomy 42e Ch 65; Bailey & Love 28e]
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