**Question:** Commonly used long bone for identification

**Core Concept:** Long bones are essential for the structure and function of the human body, especially for supporting weight, providing a surface for muscle attachment, and facilitating movement. These bones are predominantly composed of cortical bone (outer, compact shell) and spongy bone (inner, trabecular structure). Common long bones include the humerus, femur, tibia, fibula, and radius/ulna.

**Why the Correct Answer is Right:** The correct answer, **D. Radius and Ulna**, is chosen because they are paired bones that make up the forearm. These bones primarily serve to provide structural support and enable movement in the forearm region. They are essential for grasping and manipulating objects.

**Why Each Wrong Option is Incorrect:**

A. **Humerus:** Although a long bone, the humerus is not a suitable answer because it is a single bone, not a pair, making **D** the more appropriate choice for the forearm bones.

B. **Femur:** The femur is the thigh bone, not a part of the forearm and thus incorrect.

C. **Tibia and Fibula:** These bones are part of the lower leg (tibia) and ankle (fibula), not the forearm.

**Clinical Pearl / High-Yield Fact:** A common misconception in medical students is considering the radius and ulna as "opposite" bones, implying they are always on the opposite sides of the body. However, this is a misnomer; they are actually adjacent bones, with the radius on the thumb side and the ulna on the little finger side. This distinction helps in identifying these bones correctly during physical examination in clinical practice.

**Correct Answer:** **D. Radius and Ulna**