Mucins are

Correct Answer: B

Rationale: Step-by-step rationale: 1. Mucins are heavily glycosylated proteins that form a gel-like barrier on mucosal surfaces. 2. This barrier helps protect the underlying tissues from pathogens and physical damage. 3. Mucins are primarily produced by goblet cells in epithelial tissues. 4. Choice B correctly describes the function and structure of mucins. Summary: - Choice A is incorrect because not all epithelial cells produce mucins. - Choice C is incorrect as mucins are not natural antibiotics. - Choice D is incorrect as mucins are present in various mucosal surfaces, not exclusively in the respiratory tract.

Correct Answer: D

Rationale: The correct answer is D because all three choices are valid triggers for complement activation. A is correct as opsonins like MBL can bind to microbes and activate the complement system. B is correct as direct recognition of microbes can also trigger complement activation. C is correct as the formation of immune complexes can lead to complement activation through the classical pathway. Therefore, all of the above choices are triggers for complement activation.

Correct Answer: D

Rationale: The correct answer is D because vertebrate eyes have single-lens eyes where light is focused by the curved cornea and lens (A), and changing the shape of the lens allows for focusing at different distances (B). Additionally, photoreceptor cells in the retina detect light (C). Therefore, all of the above statements are true, making D the correct choice. Choice A is incorrect because it does not encompass the entire process of vision in vertebrate eyes, missing the role of changing the lens shape. Choice B is incorrect as it only addresses the lens shape without mentioning the role of the cornea in focusing light. Choice C is incorrect as it only focuses on the function of photoreceptor cells in the retina without considering the lens and cornea's role in focusing light.

Correct Answer: D

Rationale: The correct answer is D because a heart attack, also known as a myocardial infarction, involves damage to the cardiac muscle due to a blocked coronary artery. Choice A is correct as it directly relates to the definition of a heart attack. Choice B is also correct as a blocked coronary artery is the main cause of decreased blood flow to the heart muscle leading to a heart attack. Choice C is incorrect as it refers to a stroke, which is the death of brain tissue due to blocked arteries in the head, not a heart attack. Therefore, A & B (choices A and B) are the correct options, making D the correct answer.

Correct Answer: B

Rationale: The correct answer is B: Producing hormones. The immune system's main functions are to protect the body from foreign substances and repair damaged tissues. Producing hormones is not a direct function of the immune system. Hormone production is primarily regulated by the endocrine system, not the immune system. Therefore, option B is not a function of the immune system. Options A and C are incorrect as protecting the body from foreign substances and repairing damaged tissues are essential roles of the immune system. Option D is incorrect because not all functions listed are actually performed by the immune system.