Correct Answer: B

Rationale: The retinas belong to the nervous system. The retina is a layer of tissue at the back of the eye responsible for detecting light and sending visual information to the brain through the optic nerve. This process is essential for vision, which is controlled by the nervous system. Choices A, C, and D are incorrect because the retinas have no association with the skeletal, lymphatic, or circulatory systems.

Correct Answer: D

Rationale: The sebaceous glands are a component of the integumentary system, which includes the skin, hair, nails, and associated structures. Sebaceous glands are typically found throughout the skin and are responsible for producing an oily substance called sebum, which helps keep the skin and hair lubricated and moisturized. The endocrine system (Choice A) consists of glands that produce hormones, the digestive system (Choice B) is responsible for processing food, and the respiratory system (Choice C) is involved in breathing and gas exchange.

Correct Answer: A

Rationale: Gallstones become most problematic when they block a duct, such as the bile duct or cystic duct. This blockage can lead to various complications, including inflammation, infection, and even damage to the gallbladder or liver. In severe cases, a blocked duct can cause intense pain, jaundice, and other serious symptoms that require medical intervention. Choices B, C, and D are incorrect because the presence of cholesterol or bilirubin in gallstones or the process of dissolution do not necessarily indicate when gallstones become most problematic. The critical issue arises when the stones obstruct a duct, leading to complications.

Correct Answer: D

Rationale: Cilia are not part of the body's secondary defense system. They are hair-like structures primarily involved in movement and transporting substances across cell surfaces, such as in the respiratory tract for debris and pathogen removal. However, cilia do not play a direct role in the immune response mechanisms characteristic of the body's secondary defense system. Macrophages, leukocytes, and T cells are immune cells that actively participate in the secondary defense system by targeting and eliminating pathogens.

Correct Answer: C

Rationale: The correct answer is C: 26. The human foot consists of 26 bones, including tarsal bones, metatarsal bones, and phalanges. These bones collaborate to offer support, flexibility, and stability for various activities like standing, walking, and running. Choice A (6) is incorrect as it is too low for the number of bones in the human foot. Choice B (16) is incorrect as it underestimates the actual number of bones in the foot. Choice D (36) is incorrect as it overestimates the number of bones in the human foot.