This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Abdomen & Pelvis, Subject name: Anatomy
A 58-year-old postmenopausal woman is diagnosed with carcinoma of the distal gastrointestinal tract. During surgery lymph nodes from the sacral, internal iliac, and inguinal lymph node groups were removed and sent for histopathologic examination. The pathology report revealed positive cancerous cells only at the inguinal lymph nodes. Which of the following parts of the gastrointestinal tract were most likely affected? 
Select and explain. 
A. Cutaneous portion of anal canal, B. Distal rectum, C. Mucosal zone of anal canal, D. Pectinate line of anal canal
A: Cutaneous portion of anal canal
Answer: A: Cutaneous portion of anal canal 
Explanation: Lymph from the cutaneous portion of the anal canal (below the pectinate line) drains into the inguinal nodes. Lymph from most parts of the rectum and from the mucosal zone of the anal canal (above the pectinate line) drains into the internal iliac nodes. Lymph from some parts of the rectum also drains into the sacral nodes.