On axial CT of the chest at the level of the carina, the structure immediately anterior to the descending thoracic aorta and posterior to the heart is the:

• A Thoracic duct
• B Hemiazygos vein
• C Oesophagus ✓
• D Left main bronchus

Explanation

At the level of the carina on axial CT, the oesophagus lies immediately anterior to the descending thoracic aorta and posterior to the heart/pericardium. This relationship is clinically important for identifying oesophageal pathology (e.g., carcinoma invading the aorta), and for recognising mediastinal masses displacing the oesophagus. The thoracic duct runs posterior to the oesophagus. The hemiazygos is left lateral to the aorta at lower levels. The left main bronchus is more anterior and cephalic at the level of the carina.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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