**Core Concept**
The subclavian artery is a major artery that supplies blood to the upper limb. It is divided into three parts, with the first part being located proximal to the medial border of the first rib. A block at the outer border of the first rib would affect the arterial supply to the upper limb.

**Why the Correct Answer is Right**
The superior thoracic artery is a branch of the axillary artery, not the subclavian artery. It arises from the axillary artery after the first part of the subclavian artery has been blocked. This means that the superior thoracic artery does not contribute to maintaining circulation to the upper limb in the scenario described. In contrast, the subscapular artery, thyrocervical trunk, and suprascapular artery are all branches of the subclavian artery that would be affected by a block at the outer border of the first rib.

**Why Each Wrong Option is Incorrect**
**Option A:** The subscapular artery is a branch of the axillary artery, but it arises from the third part of the axillary artery, which is still supplied by the subclavian artery. Therefore, it would be affected by a block at the outer border of the first rib.
**Option C:** The thyrocervical trunk is a branch of the subclavian artery that supplies blood to the thyroid gland, larynx, and shoulder region. A block at the outer border of the first rib would affect its blood supply.
**Option D:** The suprascapular artery is also a branch of the subclavian artery that supplies blood to the shoulder region. A block at the outer border of the first rib would affect its blood supply.

**Clinical Pearl / High-Yield Fact**
In the event of a subclavian artery block, the blood supply to the upper limb can be maintained through collateral circulation, which involves the formation of new blood vessels to bypass the blockage.

**✓ Correct Answer: B. Superior thoracic artery**