Which of the following represents the first line of defense to an intruding pathogen?

In a well-designed experiment, all variables apart from the treatment should be kept constant between the control group and treatment group.

This means researchers can correctly measure the entire effect of the treatment without interference from confounding variables.

Choice B) Independent variable and dependent variable is incorrect because these are not groups but rather variables.

The independent variable is manipulated by the experimenters while the dependent variable is measured to see if it changes as a result of the manipulation.

Choice C) Experimental group and non-experimental group is incorrect because a non-experimental group is not a term used in experimental design.

The correct term for the group that does not receive the treatment is control group.

Choice D) High level and low level of the independent variable is incorrect because these are levels of the independent variable, not groups.

In an experiment, there can be multiple levels of the independent variable, but they are applied to different groups (e.g.

control group, treatment group).

The correct answer is choice B.

It decreases the activation energy required for the reaction.

A catalyst provides a new reaction pathway in which a lower activation energy is offered.

This allows more reactant molecules to collide with enough energy to surmount the smaller energy barrier, increasing the rate of reaction 2.

Choice A, It increases the activation energy required for the reaction, is not the correct answer because it describes the opposite effect of a catalyst.

Choice C, It has no effect on the activation energy required for the reaction, is not the correct answer because a catalyst does have an effect on activation energy.

Choice D, It increases the rate of reaction but has no effect on the activation energy, is not the correct answer because a catalyst increases the rate of reaction by decreasing the activation energy.

Proximal tubule

The proximal tubule is responsible for reabsorbing all the nutrients and most of the water.

Choice A is incorrect because the distal tubule is not primarily responsible for reabsorbing ions, water and nutrients.

Choice C is incorrect because the glomerulus is responsible for filtering fluid and solutes out of the blood to form a glomerular filtrate.

Choice D is incorrect because the Loop of Henle is not primarily responsible for reabsorbing ions, water and nutrients.

The correct answer is choice A.

An increase in viscosity of a fluid results in a decrease in mobility of particles.

Viscosity is the resistance of a fluid to a change in shape or movement of neighboring portions relative to one another.

It denotes opposition to flow and may be thought of as internal friction between the molecules.

Choice B is incorrect because an increase in viscosity does not affect the density of a fluid.

Choice C is incorrect because an increase in viscosity results in a decrease, not an increase, in flow rate.

Choice D is incorrect because an increase in viscosity does not affect the pressure of a fluid.

Natural killer cells.

Natural killer (NK) cells are large granular lymphocytes that are capable of destroying cells infected by viruses or bacteria and susceptible tumor cells without prior sensitization and restriction by MHC antigens.

Helper T cells (choice A) are a type of white blood cell that helps other immune cells respond to infections but do not directly kill infected or tumor cells.

B cells (choice B) are a type of white blood cell that produces antibodies to fight infections but do not directly kill infected or tumor cells.

Cytotoxic T cells (choice D) are a type of white blood cell that can kill infected or tumor cells but require prior sensitization to do so.

The correct answer is choice C. Substitution.

A substitution mutation is a type of point mutation where one base in the DNA sequence is replaced by another base.

Choice A is incorrect because a deletion mutation occurs when one or more bases are removed from the DNA sequence.

Choice B is incorrect because an insertion mutation occurs when one or more bases are added to the DNA sequence.

Choice D is incorrect because an inversion mutation occurs when a segment of DNA is reversed within the chromosome.

The correct answer is choice B.

To amplify specific regions of DNA.

PCR (polymerase chain reaction) is a laboratory technique used to make many copies of a specific region of DNA.

The goal of PCR is to make enough of the target DNA region that it can be analyzed or used in some other way.

PCR has many research and practical applications, including DNA cloning, medical diagnostics, and forensic analysis of DNA.

Choice A is incorrect because PCR does not separate DNA fragments by size. Choice C is incorrect because PCR does not sequence DNA fragments.

Choice D is incorrect because PCR does not analyze protein expression levels.

Pattern recognition receptors (PRRs) are a class of receptors that can directly recognize the specific molecular structures on the surface of pathogens.

PRRs play a crucial role in the proper function of the innate immune system and are germline-encoded host sensors, which detect molecules typical for the pathogens.

Choice B is incorrect because cytokines are not receptors but rather signaling molecules that regulate immunity.

Choice C is incorrect because chemokines are not receptors but rather signaling molecules that attract immune cells to sites of infection.

Choice D is incorrect because T cells are not receptors but rather white blood cells that assist B cells or directly kill infected cells.

The correct answer is choice C. Estrogen.

Estrogen is a steroid hormone responsible for the development of female secondary sexual characteristics such as breast development, menstrual cycle, and widening of hips.

Choice A, Progesterone, is not the correct answer because it is required to maintain pregnancy and delivery.

Choice B, Testosterone, is not the correct answer because it is a hormone produced by the testes which controls the development of male secondary sexual characteristics.

Choice D, FSH (Follicle Stimulating Hormone), is not the correct answer because it stimulates the growth of ovarian follicles in females and regulates spermatogenesis in males.

Inflammatory cytokines released during the early response to bacterial infection play a crucial role in initiating cell recruitment and local inflammation 1.

They induce the expression of adhesion molecules in endothelial cells and promote the recruitment of neutrophils to the site of inflammation 1.

Choice A is incorrect because while inflammatory cytokines may enhance phagocytosis, they do not directly disrupt the infection.

Choice B is incorrect because inflammatory cytokines do not directly attack invading pathogens.

Choice D is incorrect because inflammatory cytokines do not secrete antibodies to neutralize pathogens.