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A. Angiotensin II:Angiotensin II is commonly released during prolonged exercise because dehydration and sweating reduce plasma volume, activating the renin–angiotensin system. Its release helps constrict blood vessels and maintain blood pressure despite fluid losses. This hormone therefore supports circulatory stability during long-duration exertion such as a marathon.

B. Atrial natriuretic peptide (ANP):ANP is least likely to be released because it is normally secreted when the atria are stretched by increased blood volume. During a marathon, plasma volume typically decreases due to sweating and fluid shifts, reducing atrial stretch. With lower circulating volume, ANP secretion falls rather than rises, making it the least expected hormone in this scenario.

C. Epinephrine:Epinephrine release increases significantly during endurance events due to activation of the sympathetic nervous system. It supports cardiac output, mobilizes glucose and fatty acids, and maintains perfusion to active muscles. Elevated epinephrine is therefore a natural and necessary response to prolonged physical stress.

D. Antidiuretic hormone (ADH):ADH secretion rises during prolonged exercise because reduced blood volume and increased plasma osmolality stimulate its release. This hormone promotes water reabsorption in the kidneys, helping conserve fluid and maintain blood pressure. Its release is essential to counteract dehydration during a marathon.

E. Aldosterone:Aldosterone release increases as part of the renin–angiotensin–aldosterone system during extended physical activity. It enhances sodium and water reabsorption, supporting blood pressure and fluid balance as the runner loses electrolytes through sweat. This hormone plays a key role in maintaining volume homeostasis during endurance exercise.

A. The flu has several strains that change seasonally:Influenza viruses undergo frequent mutations, leading to new strains each season. Because memory cells from previous vaccinations may not recognize these new strains, annual vaccination is required to provide effective immunity against the current circulating strains.

B. Exposure to flu vaccine produces no memory cells from proliferating B-lymphocytes:The flu vaccine does stimulate memory B cells, but its effectiveness can be reduced due to antigenic variation in the virus. The need for seasonal vaccination is not because memory cells fail to form, but because the virus changes frequently.

C. The polio virus is substantially weaker than the flu virus:The strength or virulence of the virus is not the reason for the difference in vaccination schedules. Polio vaccines induce long-lasting immunity because the virus does not mutate rapidly like influenza.

D. The flu vaccine is substantially weaker than the polio vaccine:Vaccine potency does not explain the need for annual flu shots. The primary reason is the frequent mutation of influenza viruses, not the inherent strength or weakness of the vaccine itself.