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?