WBR0940
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| Author | PageAuthor::William J Gibson (Reviewed by Serge Korjian) |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Anatomy |
| Sub Category | SubCategory::Gastrointestinal |
| Prompt | [[Prompt::A 65-year-old man presents to his primary care physician for blood in his stools. He reports no pain with passing stool, but he states he has lost 10 pounds over the past 2 months. Physical exam and diagnostic sigmoidoscopy reveal a single mass in the distal anal canal. The mass is biopsied and shows tumor invasion of the basement membrane. Which of the following sites would be the most likely next site of spread of the tumor?]] |
| Answer A | AnswerA::Superficial inguinal lymph nodes |
| Answer A Explanation | AnswerAExp::Below the pectinate line, the rectum is drained by the superficial inguinal lymph node. |
| Answer B | AnswerB::Inferior mesenteric lymph nodes |
| Answer B Explanation | [[AnswerBExp::The mesenteric lymph nodes drain the gut. The superior mesenteric lymph nodes drain all parts of the gut proximal to the sigmoid colon. The inferior mesenteric lymph nodes drain lymph from the sigmoid colon to the rectum (above the pectinate line).]] |
| Answer C | AnswerC::Para-aortic lymph nodes |
| Answer C Explanation | AnswerCExp::The para-aortic lymph nodes drain the testes and the lower gastrointestinal tract. The superficial inguinal lymph node is a more proximal site of lymphatic drainage for the rectum than the para-aortic lymph nodes. |
| Answer D | AnswerD::Liver |
| Answer D Explanation | [[AnswerDExp::While the liver is the most common site of metastasis in colon cancer patients, cancer cells are more likely to spread to the lymphatics before metastasis. Furthermore, cancers below the pectinate line are more likely squamous cell carcinomas than colorectal adenocarcinomas.]] |
| Answer E | AnswerE::Lung |
| Answer E Explanation | [[AnswerEExp::While lung is the second most common site of metastasis in colon cancer patients, cancer cells are more likely to spread to the lymphatics before metastasis. Furthermore, cancers below the pectinate line are more likely squamous cell carcinomas than colorectal adenocarcinomas.]] |
| Right Answer | RightAnswer::C |
| Explanation | [[Explanation::The patient in this vignette most likely has anal squamous cell carcinoma (SCC). The dentate or pectinate line divides the upper 1/3 and lower 2/3 of the anal canal. The line marks the transition from columnar epithelium to squamous epithelium; and demarcates the meeting point of ectoderm (distal anal canal) and endodermal hindgut (proximal anal canal). Cancers of the anal canal are relatively uncommon and account for 2% of colorectal cancers. Anal SCC progresses from a locally confined lesion, to a cancer that invades the basement membrane, to lymphatic spread and finally metastasis. This set of steps is the basis for the TNM staging system. Because of the location of this particular tumor, the most likely site of spread from the primary site is via the lymphatics to the superficial inguinal lymph nodes. Above the pectinate line, the tumor would most likely be a colorectal adenocarcinoma and would first reach the mesenteric lymph nodes. Educational Objective: The most likely route of spread of a distal rectal tumor (squamous cell carcinoma most common) is via the lymphatics to the superficial inguinal lymph nodes. |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Lymph node, WBRKeyword::Anus, WBRKeyword::Anal, WBRKeyword::Colon, WBRKeyword::Cancer, WBRKeyword::Rectum, WBRKeyword::Lymphatic, WBRKeyword::Lymph |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |