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45. The correct answer is (E). Erythropoietin is a hormone formed in the kidney. This hormone stimu-
lates red blood cell production (red blood cells are also called erythrocytes). Erythropoietin does not
regulate the white blood cell population.
46. The correct answer is (B). Maximal growth acceleration occurs between the fourth and sixth hours.
The diagram depicts the familiar “S-shaped” growth curve. At a low population density, the bacteria
initially multiply rapidly, then reproduction slows until a steady state is reached. Growth acceleration
occurs between hours 0–6, with maximal acceleration between the fourth and sixth hours. Between
the sixth and tenth hours, deceleration is actually occurring. The steady state is reached between the
tenth and twelfth hours.
47. The correct answer is (B). Passive immunity is the term used to describe the injection of preformed
antibodies. That is, the antibodies are synthesized elsewhere (laboratory synthesis, pooled antiserum)
but are given to the patient so an immediate immune response can be obtained. Active immunity
occurs when an individual mounts an immune response, synthesizes antibodies, reacts to a foreign
stimulus (the antigen). Active immunity occurs as a result of being infected or by receiving a vaccine.

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Lesson 9
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ARCO
■ SAT II Subject Tests
48. The correct answer is (A). The mammary glands are ectodermal derivatives. The thyroid gland, the
liver, and the Eustachian tube are derived from endoderm. Mesodermal derivatives include the heart,
the kidneys, and the adrenal cortex.
49. The correct answer is (C). The first dinosaurs inhabited the earth during the Mesozoic era. The age
of mammals is classified as early Cenozoic. Fish, amphibians, and reptiles each have origins tracing
back to the Paleozoic era. Precambrian life consisted of primitive marine organisms. Within the Paleo-
zoic era is the Silurian period (i.e., Silurian is a period, not an era).