**Question:** All of the following calcify except –

A. Calvarium (skull bones)
B. Vertebrae
C. Osteons (bone structure)
D. Articular cartilage

**Correct Answer:** D. Articular cartilage

**Core Concept:**
Calcification is the process by which minerals, particularly calcium and phosphorus, accumulate within a tissue or structure, leading to hardening and increased mineral content. In the context of the human body, calcification can occur in various tissues and structures.

**Why the Correct Answer is Right:**
Articular cartilage, also known as synovial cartilage, is a smooth, hydrated connective tissue that covers the ends of long bones in synovial joints. While articular cartilage does contain calcium and phosphate ions, it does not undergo calcification. This is because calcification results in hardening and loss of flexibility, which is detrimental for the smooth movement in joints. Articular cartilage maintains its elasticity and lubrication properties through the presence of ground substance, proteoglycans, and water.

**Why Each Wrong Option is Incorrect:**

A. Calvarium (skull bones) is a correct answer because the skull bones undergo significant calcification during development and maintain their mineral content throughout life. Calvarium is composed of dense, compact bone that is rich in calcium and phosphate, making it non-living and rigid.

B. Vertebrae also undergo calcification during development, but they possess a unique structure called the annulus fibrosus and the nucleus pulposus. These structures, which are rich in calcium and phosphate, provide rigidity and shock absorption to the vertebrae, allowing for spinal movement and protection.

C. Osteons, also known as Haversian systems, are mineralized structures found within bone tissue. They are composed of collagen fibers, hydroxyapatite crystals, and bone cells, which collectively provide strength and rigidity to the bone. Osteons undergo continuous remodeling, ensuring bone health and maintaining its structural integrity.

**Core Concept:**
Calcium and phosphate are essential ions for bone and joint health, as they contribute to bone mineralization, strength, and rigidity.

**Why Each Wrong Option is Incorrect:**

Option A (Calvarium) and C (Osteons) are correct because they undergo calcification, which is vital for their structure and function.

Option B (Vertebrae) is incorrect because, although vertebrae undergo calcification, they possess specialized structures like annulus fibrosus and nucleus pulposus that enable flexibility and spinal protection, not rigidity.

Option D (Articular cartilage) is incorrect because, despite containing calcium and phosphate, it does not undergo calcification. Articular cartilage maintains its elasticity and lubrication through ground substance, proteoglycans, and water, allowing for smooth joint movement and protection.