The most common deformity of the spine is an abnormal lateral curvature called:

Choice A reason: The medullary cavity is found in the diaphysis of long bones and contains bone marrow. It is not a feature of spongy bone.

Choice B reason: Lamellar bone refers to mature bone with organized collagen fibers. While spongy bone can be lamellar, this does not define its structural organization.

Choice C reason: Osteons, or Haversian systems, are cylindrical structures found in compact bone, not spongy bone.

Choice D reason: Trabeculae are the defining structural units of spongy bone. They are lattice-like networks of bone tissue that provide strength while minimizing weight and house bone marrow between their spaces.

Choice A reason: Tendons connect muscles to bones and transmit force during contraction. They do not act as cushions or reduce friction.

Choice B reason: Bursae are the correct answer. These are small fluid-filled sacs lined with synovial membrane that reduce friction between moving structures such as tendons and bones or skin and bones. They are commonly found near joints like the shoulder, elbow, and knee.

Choice C reason: Menisci are fibrocartilaginous pads found in certain joints like the knee. They help distribute weight and improve joint stability but are not fluid-filled sacs.

Choice D reason: Joint cavities are spaces within synovial joints that contain synovial fluid. While they allow movement, they are not separate cushioning structures like bursae.

Choice E reason: Ligaments connect bones to other bones and stabilize joints. They do not serve as cushions or reduce friction.