I am going to answer the sample USMLE Step 1 Questions with Explanation. These questions are available at USMLE.org
A 65-year-old woman comes to the office for a follow-up examination 1 year after she underwent operative resection of the right colon and chemotherapy for stage III colon cancer. She reports fatigue. Physical examination shows no abnormalities. A staging CT scan of the chest and abdomen shows five new 2- to 3-cm masses in the liver and both lungs. This patient’s cancer most likely spread to the lungs via which of the following structures?
(A) Inferior mesenteric vein
(B) Inferior vena cava
(C) Left colic vein
(D) Middle colic artery
(E) Pulmonary vein
(F) Superior mesenteric artery
(G) Superior vena cava
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Answer
Colon cancer typically spreads through the lymphatic system or the blood. In this patient, the cancer has metastasized to the liver and lungs, suggesting a hematogenous (blood-borne) spread.
- The colonic veins, including the right colic vein, drain into the superior mesenteric vein, which then joins the splenic vein to form the portal vein. This portal system carries blood from the gastrointestinal tract to the liver, which is often the first site of metastasis for colon cancer.
- Once cancer spreads to the liver, it can then spread to other organs, such as the lungs. The metastatic cells in the liver can enter the systemic circulation via the hepatic veins, which drain into the inferior vena cava (IVC). The IVC then carries the blood back to the right side of the heart, from where it is pumped to the lungs via the pulmonary arteries. This pathway explains how the cancer reaches the lungs.
The correct answer is B