This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Blood Vessels of Thorax, Subject name: Anatomy
1st posterior intercostal artery is a branch of- 
Select and explain. 
A. Aorta, B. Superior intercostal artery, C. Internal mammary artery, D. Bronchial artery
B: Superior intercostal artery
Answer: B: Superior intercostal artery 
Explanation: Ans. is 'b' i.e., Superior intercostal artery o Each of upper nine intercostal spaces (1 to 9) have one posterior and two anterior intercostal arteries. The 10thand 11th spaces have one posterior intercostal artery (no anterior intercostal artery)o Anterior intercostal arteries for upper six spaces (two in each space) arise from internal thoracic or internal mammary artery. For 7th to 9th spaces, these are branches of musculophrenic artery (terminal branch of internal thoracic artery).o 1st and 2nd posterior intercostal arteries are branches of superior intercostal artery (a branch of costocervical trunk from 2nd part of subclavian artery). Lower nine (3rd to 11th) posterior intercostal arteries are branches of descending thoracic aorta. Right posterior intercostal arteries are longer than the left.