4- The basic unit of collagen microfibrils: A. Is tropocollagen. B. Is sarcomere. C. Is produced by fibroblast. D. Is produced by osteoclast. E. Is abnormal in Duchenne muscular dystrophy. F. Is abnormal in scurvy. G. Is tropoelastin

The basic unit of collagen microfibrils is tropocollagen, so the correct answer is A. Tropocollagen.

Collagen microfibrils are produced by fibroblasts, not osteoclasts, so the correct answer is also C. Is produced by fibroblast.

Scurvy is a disease caused by a deficiency of vitamin C, which is necessary for the synthesis of collagen. Therefore, in scurvy, collagen microfibrils can be abnormal, so the correct answer is also F. Is abnormal in scurvy.

The other options are incorrect. Sarcomere is the basic unit of a muscle's contractile apparatus, not collagen microfibrils, so B is incorrect. Duchenne muscular dystrophy is a genetic disorder characterized by progressive muscle degeneration, not abnormal collagen, so E is incorrect. Tropoelastin is the basic unit of elastin, not collagen, so G is incorrect.