The role of the motor neuron at the neuromuscular junction is to:

Correct Answer: B

Rationale: At the neuromuscular junction, the motor neuron releases acetylcholine into the synaptic cleft, stimulating the muscle fiber to contract. Calcium is stored in the sarcoplasmic reticulum, ATP is produced in mitochondria, and muscle fibers contract, not neurons, making 'Release acetylcholine' correct.

Correct Answer: D

Rationale: Short bones, like carpals and tarsals, have roughly equal length and width, providing stability and support. Flat bones are thin and broad, irregular bones have complex shapes, and long bones are longer than wide, making 'short' the correct classification.

Correct Answer: D

Rationale: Compact bone is organized into cylinder-shaped units called osteons (or Haversian systems—both terms are synonymous), containing lamellae and a central canal. Trabeculae are in spongy bone, not compact, so 'Both A and C are correct' reflects the interchangeable terminology.

Correct Answer: B

Rationale: Yellow bone marrow, found in the medullary cavity of adult long bones, is primarily fat tissue (adipose), serving as an energy reserve. Cartilage is at joints, and red/white blood cells are in red marrow, making 'fat tissue' correct.

Correct Answer: A

Rationale: Endochondral ossification follows: 1) bone collar forms around the cartilage model, 2) primary ossification center forms in the diaphysis, 3) secondary centers form in the epiphyses, 4) epiphyseal plate ossifies, closing growth. Thus, '1-2-3-4' is the correct order.