In co-arctation of aoa of the post -ductal type, all of the following blood vessels take pa in forming the collaterals execpt:
**Core Concept**
Co-arctation of the aorta refers to a congenital narrowing of the aortic isthmus, typically located after the origin of the left subclavian artery and before the ductus arteriosus. In post-ductal co-arctation, the ductus arteriosus is patent and can contribute to the formation of collateral blood vessels.
**Why the Correct Answer is Right**
In post-ductal co-arctation, the ductus arteriosus, internal thoracic arteries, and inferior phrenic arteries form the collateral circulation to bypass the narrowed segment of the aorta. The ductus arteriosus connects the pulmonary artery to the aorta, allowing blood to flow from the pulmonary artery to the aorta and providing a pathway for collateral circulation. The internal thoracic arteries, which arise from the subclavian arteries, form anastomoses with the posterior intercostal arteries to create an additional collateral pathway. The inferior phrenic arteries, which arise from the aorta, form anastomoses with the superior phrenic arteries to contribute to the collateral circulation.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the left subclavian artery contributes to the formation of collateral vessels in co-arctation of the aorta. The internal thoracic artery, which arises from the left subclavian artery, forms anastomoses with the posterior intercostal arteries to create a collateral pathway.
* **Option B:** This option is incorrect because the right subclavian artery does not typically contribute to the formation of collateral vessels in post-ductal co-arctation. The primary collateral pathways in post-ductal co-arctation involve the ductus arteriosus, internal thoracic arteries, and inferior phrenic arteries.
* **Option C:** This option is incorrect because the superior intercostal artery, which arises from the costocervical trunk, does not typically contribute to the formation of collateral vessels in post-ductal co-arctation. The posterior intercostal arteries, which arise from the aorta, form anastomoses with the internal thoracic arteries to create a collateral pathway.
**Clinical Pearl / High-Yield Fact**
In post-ductal co-arctation, the presence of a patent ductus arteriosus allows for the formation of collateral blood vessels, which can help to maintain blood flow to the lower body. However, the collateral circulation can also lead to increased blood flow to the lungs, which can contribute to pulmonary hypertension.
**Correct Answer: B.**