A 46-year-old man presents with an ulcerating squamous cell carcinoma arising from the anal margin, entirely below the level of the dentate line. Clinically there are no palpable groin nodes. To which nodal basin does this tumour first drain?

• A Superior mesenteric nodes along the inferior mesenteric artery
• B Internal iliac (hypogastric) nodes
• C External iliac nodes
• D Superficial inguinal nodes ✓

Explanation

Lymphatic drainage of the anal canal follows the dentate line. Above it, drainage runs with the superior rectal vessels to the inferior mesenteric chain and laterally to the internal iliac nodes. Below the dentate line, drainage is to the superficial inguinal nodes, which are also the first echelon for anal margin tumours and for cancers extending to the perianal skin. This is why groin examination and node sampling matter in low anal lesions, while external iliac nodes are not in the primary pathway.

Reference: Gray's Anatomy, 42nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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