A 68-year-old man presents with progressive fatigue and dyspnea on exertion over 4 months. Laboratory studies reveal hemoglobin 8.2 g/dL, MCV 72 fL, serum ferritin 12 ng/mL, and transferrin saturation 18%. Colonoscopy reveals the lesion marked **B** in the diagram — a polypoid, exophytic mass in the cecum with biopsy-confirmed adenocarcinoma. Which of the following best explains why iron-deficiency anemia is the presenting feature of right-sided colonic adenocarcinoma rather than acute obstruction?

Question

Accepted Answer

B. The wide cecal lumen accommodates bulky polypoid masses without early luminal narrowing, allowing chronic occult bleeding to manifest as iron deficiency before obstruction occurs. ## Why option 1 is correct

Right-sided colonic adenocarcinomas (cecum, ascending colon) grow as polypoid, exophytic, bulky fungating masses into the **wide cecal lumen**. This anatomical feature allows the tumor to grow substantially without causing luminal narrowing or mechanical obstruction. Instead, the tumor surface ulcerates and bleeds chronically into the stool, causing occult gastrointestinal blood loss over weeks to months. This chronic bleeding depletes iron stores, presenting as iron-deficiency anemia (microcytic, hypochromic, low ferritin, low transferrin saturation) — a classic presentation that mandates bidirectional endoscopy (colonoscopy + EGD) in all men and postmenopausal women regardless of menstrual status, because right-sided colon cancer is the dominant concern (Sabiston Textbook of Surgery 21e; NCCN Colon Cancer 2024).

## Why each distractor is wrong

- **Option 2**: While right-sided tumors may invade deeply, transmural inflammation and impaired ileal iron absorption are not the primary mechanism of iron deficiency in right-sided CRC. The anemia results from chronic bleeding, not malabsorption.
- **Option 3**: Although polypoid tumors do ulcerate and bleed, this option incorrectly implies that annular left-sided tumors obstruct "before significant blood loss." Left-sided tumors present with obstruction and hematochezia/change in bowel habit, not anemia as the primary feature — the distinction is anatomical (wide vs. narrow lumen), not inherent tumor biology.
- **Option 4**: The ileocolic vessels are not inherently more friable than left colic vessels. The key difference is the **wide cecal lumen** that permits chronic bleeding without early obstruction, not vascular fragility.

**High-Yield:** Right-sided CRC = polypoid + wide lumen + chronic occult bleeding + iron-deficiency anemia; left-sided CRC = annular + narrow lumen + early obstruction + hematochezia.

[cite: Sabiston Textbook of Surgery 21e; NCCN Colon Cancer 2024; USPSTF CRC Screening 2021]

Answer

A. Right-sided tumors preferentially invade the muscularis propria, causing transmural inflammation that impairs iron absorption in the terminal ileum

Answer

C. Polypoid right-sided tumors are more likely to ulcerate and bleed than annular left-sided tumors, which obstruct before significant blood loss

Answer

D. The ileocolic blood supply is more friable than the left colic vessels, predisposing right-sided tumors to earlier and more profuse hemorrhage

Explanation

Why option 1 is correct

Why each distractor is wrong

Practice similar questions