Question:
The aortic knob silhouette on a chest X-ray (PA view) is obliterated by consolidation of which portion of the lung?
The aortic knob silhouette on a chest X-ray (PA view) is obliterated by consolidation of which portion of the lung?
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Which lung segment is draped directly against the aortic arch high on the left mediastinum?
Updated On: Jun 24, 2026
- Apicoposterior segment of left upper lobe
- Superior lingular segment of left upper lobe
- Apical segment of left lower lobe
- Inferior lingular segment of left upper lobe
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The Correct Option is A
Solution and Explanation
Step 1: Understand the silhouette sign.
The silhouette sign states that an intrathoracic opacity, if in anatomical contact with a border of the heart or aorta, will obliterate that border. The lung tissue must be adjacent to the structure for its margin to be lost.
Step 2: Locate the aortic knob.
The aortic knob (arch) lies at the upper left mediastinal border, at a relatively high posterior level in the chest.
Step 3: Match the adjacent lung segment.
The apicoposterior segment of the left upper lobe lies in direct contact with the aortic arch. Consolidation here removes the sharp outline of the aortic knob (loss of the aortic knob silhouette).
Step 4: Exclude the others.
The lingular segments lie lower and more anterior, abutting the left heart border (their loss blurs the left cardiac border, not the aortic knob). The apical segment of the left lower lobe is posterior and lower, not in contact with the aortic arch.
Hence the answer is the apicoposterior segment of the left upper lobe.
The silhouette sign states that an intrathoracic opacity, if in anatomical contact with a border of the heart or aorta, will obliterate that border. The lung tissue must be adjacent to the structure for its margin to be lost.
Step 2: Locate the aortic knob.
The aortic knob (arch) lies at the upper left mediastinal border, at a relatively high posterior level in the chest.
Step 3: Match the adjacent lung segment.
The apicoposterior segment of the left upper lobe lies in direct contact with the aortic arch. Consolidation here removes the sharp outline of the aortic knob (loss of the aortic knob silhouette).
Step 4: Exclude the others.
The lingular segments lie lower and more anterior, abutting the left heart border (their loss blurs the left cardiac border, not the aortic knob). The apical segment of the left lower lobe is posterior and lower, not in contact with the aortic arch.
Hence the answer is the apicoposterior segment of the left upper lobe.
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