## Why option 1 is correct The Pemberton sign is a classic clinical sign of thoracic inlet obstruction caused by a substernal or retrosternal goiter. When the arms are raised above the head, the thoracic inlet is mechanically narrowed and pulled upward. In a patient with a substernal goiter, this maneuver causes the enlarged thyroid to compress both the subclavian and jugular veins (producing facial plethora and jugular distension) and simultaneously compress the trachea (producing inspiratory stridor). This is the pathognomonic mechanism described in Bailey & Love 28e Ch 50 for identifying a goiter with thoracic inlet involvement. ## Why each distractor is wrong - **Option 2**: Increased intrathoracic pressure from forced inspiration is a secondary consequence, not the primary mechanism. The sign occurs due to mechanical compression of vascular and airway structures by the goiter itself, not from respiratory mechanics alone. - **Option 3**: Thyroid hormone-induced hypercoagulability is not the mechanism of Pemberton sign. The sign reflects mechanical obstruction, not a thrombotic or coagulation phenomenon. Graves disease and toxic goiter do not cause hypercoagulability as a primary pathophysiology. - **Option 4**: Lymphatic obstruction from follicular hyperplasia does not explain the acute onset of facial plethora and stridor with arm elevation. Lymphatic obstruction would produce gradual, persistent facial edema rather than a dynamic sign triggered by postural change. **High-Yield:** Pemberton sign = facial plethora + jugular distension + inspiratory stridor on arm elevation = substernal goiter compressing thoracic inlet vessels and trachea. [cite: Bailey & Love 28e Ch 50]
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