What is the definition of a gene?

Correct Answer: B

Rationale: The correct answer is B: a length of DNA that codes for a protein. Genes are specific sequences of DNA that encode instructions for building proteins. This is the fundamental role of genes in the process of protein synthesis. Choice A is incorrect because not all DNA in a cell controls metabolic reactions; some DNA is non-coding. Choice C is incorrect as it refers to the entire nucleus and its contents, not just the gene. Choice D is incorrect as genes do not solely determine the inheritance of sex; they also code for other traits. In summary, the definition of a gene as a segment of DNA that codes for a protein aligns with the known function and role of genes in genetics and molecular biology.

Correct Answer: D

Rationale: The correct answer is D: red supergiant → supernova → black hole. This sequence is part of the life cycle of a very large mass star. Red supergiants are massive stars in the later stages of their evolution. When a red supergiant goes supernova, it explodes in a massive stellar explosion, leaving behind a remnant that can collapse into a black hole due to the immense gravitational forces. Choices A and B are incorrect because planetary nebulae form from the outer layers of lower mass stars, not very large mass stars. Choice C is incorrect as neutron stars are more likely to form from the remnants of less massive stars.

Correct Answer: A

Rationale: The correct answer is A. Eicosanoids are not true hormones because they act locally, not on distal organs like true hormones. Eicosanoids are produced and released in response to specific stimuli at the site of action, affecting nearby cells. This local action distinguishes them from true hormones, which are produced in one part of the body and travel through the bloodstream to distant target organs to elicit a response. Choices B, C, and D are incorrect because eicosanoids do have target organs (B), mediate inflammatory reactions (C), and are lipid in nature (D), but these characteristics do not define them as hormones.

Correct Answer: D

Rationale: The correct answer is D: cholecalciferol. The skin produces cholecalciferol, also known as vitamin D, in response to sunlight exposure. This hormone plays a crucial role in calcium absorption and bone health. Erythropoietin (A) is produced by the kidneys to stimulate red blood cell production. Melatonin (B) is produced by the pineal gland to regulate sleep-wake cycles. Renin (C) is produced by the kidneys to regulate blood pressure. Therefore, the skin does not produce erythropoietin, melatonin, or renin, making them incorrect choices.

Correct Answer: D

Rationale: The correct answer is D: Left renal vein. The left suprarenal vein drains into the left renal vein because they are anatomically close and share a common blood supply. The left suprarenal vein carries blood from the left adrenal gland, while the left renal vein receives blood from the left kidney. This anatomical proximity allows for efficient drainage of blood from the adrenal gland into the renal vein for further circulation. Choices A, B, and C are incorrect because the left suprarenal vein does not typically drain into the superior mesenteric vein, inferior hemiazygos, or inferior vena cava.