Final answer:
The discoloration and swelling from a muscle strain are caused by ruptured blood vessels that lead to bruising and fluid accumulation in the tissue. The correct option is D.
Step-by-step explanation:
The discoloration and swelling of a muscle strain is due to ruptured blood vessels. When a muscle strain occurs, some of the blood vessels within the muscle tissue may break, leading to internal bleeding and the characteristic bruising seen on the skin.
This also results in swelling due to the accumulation of blood and other fluids in the muscle tissue.
The correct answer to the provided question is D. ruptured blood vessels.
Tendons and myofibrils are not directly responsible for the discoloration, and while severed nerves may cause additional symptoms, they are not the reason for swelling and discoloration linked to a muscle strain. The correct option is D.