What is the role of tropomyosin in skeletal muscles?

Choice A reason: Ossification of the ends of long bones (epiphyses) does not necessarily take twice as long as the shaft (diaphysis). The timing varies depending on the bone and developmental stage. The shaft begins ossifying earlier via the primary ossification center, typically during fetal development, while the ends ossify later via secondary centers, often after birth. However, the duration is not uniformly double.

Choice B reason: Intramembranous ossification is responsible for forming flat bones such as the skull and clavicle. The ends of long bones undergo endochondral ossification, not intramembranous. Therefore, this choice incorrectly associates the process with the wrong ossification type.

Choice C reason: The ends of long bones ossify through secondary ossification centers, which appear after birth. These centers are responsible for forming the epiphyses and are distinct from the primary ossification centers in the shaft. This is the correct and scientifically supported answer.

Choice D reason: Ossification of the diaphysis (shaft) begins earlier than the epiphyses. The primary ossification center in the shaft typically appears during fetal development, while secondary centers in the ends appear postnatally. Thus, this statement is factually incorrect.

Choice A reason: The metaphysis is the region between the diaphysis and epiphysis, often containing the growth plate in children. It is not the shaft of the bone.

Choice B reason: The epiphysis refers to the ends of long bones, which articulate with adjacent bones. A fracture here would not be considered a shaft fracture.

Choice C reason: The diaphysis is the correct answer. It is the central shaft of a long bone, composed primarily of compact bone and housing the medullary cavity. Fractures in this region are common in long bones like the femur and humerus.

Choice D reason: The periosteum is a fibrous membrane covering the outer surface of bones. While it may be affected in a fracture, it is not the structural region referred to as the shaft.

Choice E reason: The endosteum lines the inner surface of the bone, including the medullary cavity. It plays a role in bone remodeling but is not the shaft itself