## Core Concept
The question tests knowledge of upper limb anatomy, specifically muscles that act across multiple joints. The **biceps brachii**, **triceps brachii**, and **brachialis** are key muscles in the upper limb, but only one fits the description of spanning across both the shoulder and elbow joints.

## Why the Correct Answer is Right
The correct answer, **B. biceps brachii**, has two parts: the long head originates from the supraglenoid tubercle of the scapula (shoulder joint), and the short head originates from the coracoid process of the scapula. Both heads converge to form a single muscle that inserts into the radial tuberosity and bicipital aponeurosis, thus spanning across the shoulder and elbow joints. This unique anatomy allows the biceps brachii to contribute to movements at both joints, such as flexion of the elbow and stabilization of the shoulder joint.

## Why Each Wrong Option is Incorrect
- **Option A:** The **triceps brachii** primarily acts on the elbow joint, originating from the posterior surface of the humerus and inserting into the olecranon of the ulna. It does not span across the shoulder joint.
- **Option C:** The **brachialis** muscle primarily flexes the elbow and does not have any attachments that would allow it to act across the shoulder joint.
- **Option D:** Without a specified muscle, this option cannot be directly evaluated, but given that **B. biceps brachii** is the correct answer, any other muscle not described here would not meet the criteria.

## Clinical Pearl / High-Yield Fact
A key clinical point to remember is that the **long head of the biceps brachii** tendon can be a source of shoulder pain and is often involved in rotator cuff pathologies. Tendinopathy or rupture of this tendon can lead to significant disability and may require surgical intervention.

## Correct Answer: B. biceps brachii