Somatostatin secreted by the pancreas is considered paracrine because it acts on __________.

Correct Answer: C

Rationale: The correct answer is C because somatostatin acts on nearby cells other than those that secrete it, which is the definition of paracrine signaling. This type of signaling involves the release of signaling molecules that affect neighboring cells within the same tissue or organ. Choices A and B are incorrect because paracrine signaling does not target the cells that secrete the signaling molecule or distant target organs. Choice D is incorrect because the hypothalamus is not typically the target of somatostatin action in the pancreas.

Correct Answer: a

Rationale: The correct answer is A: This muscle is optimized for endurance training. Dark (red) muscle contains more myoglobin, which stores oxygen and enhances endurance performance. This muscle type is rich in mitochondria, allowing for sustained energy production. Choice B is incorrect because dark muscle actually has more mitochondria. Choice C is incorrect because dark muscle contains more slow-twitch fibers, which are fatigue-resistant and contribute to endurance. Choice D is incorrect because dark muscle predominantly consists of slow-twitch fibers, making it optimized for endurance rather than speed.

Correct Answer: C

Rationale: The correct answer is C: High plasma levels of glucose. When blood glucose levels rise after a meal, pancreatic beta cells sense this increase and release insulin to facilitate glucose uptake into cells for energy production or storage. This is known as the glucose-stimulated insulin release mechanism. Low plasma levels of calcium (choice A), high plasma levels of potassium (choice B), and low blood volume (choice D) do not directly stimulate insulin release and are unrelated to the regulation of insulin secretion.

Correct Answer: C

Rationale: Estrogen, progesterone, and testosterone are indeed secreted by the gonads. Estrogen and progesterone are primarily produced by the ovaries in females, while testosterone is mainly produced by the testes in males. These hormones play crucial roles in reproductive functions and secondary sexual characteristics. Choices A and B are incorrect because testosterone induces virilization (masculinizing effects) while estrogen and progesterone are more associated with feminizing effects. Choice D is incorrect because estrogen, progesterone, and testosterone are not secreted by the adenohypophysis, which mainly produces and releases other hormones such as growth hormone and prolactin.

Correct Answer: B

Rationale: Rationale: 1. Testosterone is an androgen hormone primarily produced in the testes. 2. Androgens are responsible for male characteristics and reproductive functions. 3. Progesterone is a female sex hormone, not an androgen. 4. Estrogen is also a female sex hormone, not an androgen. 5. Aldosterone is a mineralocorticoid hormone, not an androgen. Summary: Testosterone is the correct answer as it is a male sex hormone with androgenic properties. Progesterone, estrogen, and aldosterone are not androgens and do not exhibit the same functions as testosterone.