The Costoclavicular triangle is bounded by which structures?

• A. Anterior middle third of the clavicle; posterior medial first rib; posterior lateral upper border of the scapula
• B. Anterior border of the sternocleidomastoid; first rib; posterior border of the scapula
• C. Anterior surface of the clavicle; middle third of the first rib; anterior border of the scapula
• D. Subcoracoid space beneath the pectoralis minor

Answer: (A)

The costoclavicular triangle is the small space between the clavicle and the first rib where the subclavian vessels and the lower part of the brachial plexus lie. It is defined by three boundaries: the anterior boundary is the middle third of the clavicle, the posterior boundary is the medial part of the first rib, and the posterior boundary is the posterior (lateral) portion of the upper border of the scapula. This triangular region is clinically important because compression or injury here can affect the subclavian vessels and lower brachial plexus, contributing to thoracic outlet syndrome or vascular/nerve issues. The other options point to different neck or shoulder spaces (for example, the sternocleidomastoid border relates to another neck region, the subcoracoid space lies under the pectoralis minor), so they don’t describe the costoclavicular triangle.