**Question:** Secondary retrosternal goitre derives its blood supply from:

**Core Concept:** Retrosternal goitre refers to a thyroid gland enlargement extending into the mediastinum, typically due to primary thyroid disease spreading to the mediastinum. Secondary retrosternal goitre is an extension of primary thyroid disease into the mediastinum, which receives its blood supply from the superior thyroid artery, a branch of the external carotid system.

**Why the Correct Answer is Right:** The correct answer (C) is based on the understanding of blood supply in the thyroid gland anatomy. The thyroid gland is supplied by two main arteries: the superior thyroid artery and the inferior thyroid artery, both of which are branches of the external carotid system (a part of the systemic circulation). When a thyroid disease extends into the mediastinum, the retrosternal goitre receives blood supply from the superior thyroid artery, which is a part of the external carotid system.

**Why Each Wrong Option is Incorrect:**

A. The thyroid gland is primarily supplied by the external carotid system, but retrosternal goitre receives its blood supply from the external carotid system, which is incorrect for a retrosternal goitre.

B. The thyroid gland receives blood supply from both internal and external carotid systems, but retrosternal goitre derives its blood supply only from the external carotid system, making this option incorrect.

D. The thyroid gland is supplied by both the external and internal carotid systems, but retrosternal goitre derives its blood supply from the external carotid system alone.

**Clinical Pearl:** Understanding blood supply in thyroid anatomy is crucial for differentiating between primary and secondary thyroid diseases and recognizing the extent of disease involvement. A retrosternal goitre receiving blood supply from the external carotid system suggests a secondary involvement.

**Correct Answer:** C. The thyroid gland receives blood supply from the external carotid system, and retrosternal goitre derives its blood supply from the external carotid system.