Correct Answer: D

Rationale: The correct answer is the Achilles tendon. The Achilles tendon is a strong tendon located at the back of the human leg, connecting the calf muscles to the heel bone. This positioning makes it a posterior feature, situated at the rear of the leg. The patella, also known as the kneecap, is positioned at the front of the leg, making it an anterior feature. The quadriceps sartorius are muscles located in the front of the thigh, not at the back of the leg. Additionally, the triceps brachii is a muscle found in the upper arm, not in the leg, making it an incorrect choice.

Correct Answer: D

Rationale: Melatonin is the hormone produced by the pineal gland. It plays a crucial role in regulating the sleep-wake cycle and is instrumental in maintaining the body's circadian rhythm. Melatonin production is triggered by darkness and inhibited by light, aiding in signaling the body when it is time to sleep. Corticosteroid, oxytocin, and prolactin are hormones produced by other glands in the body and are not associated with the pineal gland. Therefore, they are incorrect choices for this question.

Correct Answer: B

Rationale: The correct statement is that skeletal muscle attaches to bones through tendons, enabling movement. Tendons connect muscles to bones and allow the force generated by muscle contraction to be transmitted to the bones, facilitating voluntary movements. Choice A is incorrect because skeletal muscle typically makes up around 40-50% of an individual's body weight, not a fixed 40%. Choice C is incorrect as muscle contraction plays a vital role in temperature regulation by generating heat. Choice D is incorrect as skeletal muscles may have constant low-level contractions even at rest, known as muscle tone.

Correct Answer: A

Rationale: Selenium is the correct answer. It is considered an antioxidant mineral that plays a crucial role in protecting cells from damage caused by free radicals. Selenium is an essential component of the body's antioxidant defense system, promoting overall health and well-being. Calcium (Choice B), Sulfur (Choice C), and Iron (Choice D) are not typically classified as antioxidant minerals. While these minerals have other important functions in the body, they do not primarily act as antioxidants like selenium.

Correct Answer: C

Rationale: Trypsin is an enzyme that specifically breaks down proteins into smaller peptides by hydrolyzing peptide bonds. It is a key enzyme involved in the digestion of proteins in the body, particularly in the small intestine where it helps in the final stages of protein digestion. Choices A, B, and D are incorrect because trypsin does not break down fats, starch, or nucleic acids; its primary function is the digestion of proteins.