Explanation:

- The vertebrae are part of the skeletal system, providing structure and protection.

- Not directly related to the kidney; they're part of the urinary system.

- Unrelated to the lungs, which belong to the respiratory system.

- Has no direct link with the heart, part of the circulatory system.

- Spinal cord is surrounded by the vertebrae, offering protection and support; part of the central nervous system.

Correct Answer: Option 4 - Spinal cord