45. Within a few weeks of treatment with the drug 3TC, a patient’s HIV population consists entirely of 3TC-resistant viruses. What is the best explanation?
(A) HIV has the ability to change its surface proteins and resist vaccines.
(B) The patient must have become reinfected with 3TC-resistant viruses.
(C) HIV began making drug-resistant versions of reverse transcriptase in response to the drug.
(D) A few drug-resistant viruses were present at the start of treatment, and natural selection increased their frequency. 
(E) Some viruses developed drug resistance and then passed their function.

41. In a neuroscience lab, a student increased the concentration of K+ outside a neuron from 5 mM to 10 mM, while maintaining other ion concentrations as they were. What would happen to the neuron’s membrane potential? (A) The membrane potential would become more negative. (B) The membrane potential would become less negative. (C) The membrane potential would remain the same. (D) The membrane potential would be closer to the K+ equilibrium potential. (E) There would be no potential difference across the membrane.

42. Which of the following is not true about helper T cells? (A) They function in both cell-mediated and humoral immune responses. (B) They bear surface CD4 molecules. (C) They recognize polysaccharide fragments presented by infected cells. (D) They are subject to infection by HIV. (E) When activated, they secrete cytokines that stimulate other lymphocytes.

43. Which of the following statements about vision is not true? (A) Rods are more light-sensitive than cones and are responsible for night vision. (B) Color vision results from the presence of three subclasses of cones in the retina. (C) Perception of visual information takes place in the cerebral cortex. (D) All information from the left eye goes to the right visual cortex and all information from the right eye goes to the left visual brain. (E) The ciliary muscles relax if you look away from this page and focus your eyes on a distant object and the lens is flattened.

44. Which of the following sensory receptors is incorrectly paired with its category? (A) Photoreceptor — rod cell in retina (B) Nociceptor — muscle spindle (C) Chemoreceptor — taste receptor in tongue (D) Mechanoreceptor — hair cell in cochlea (E) Electroreceptor — hair cell in ampulla of Lorenzini

46. A founder event favors microevolution in the founding population mainly because ________. (A) mutations are more common in a new environment (B) a small founding population is subject to extensive sampling error in the composition of its gene pool (C) the new environment is likely to be patchy, favoring diversifying selection (D) gene flow increases (E) members of a small population tend to migrate

47. Which of the following descriptions are correct? 1. Green algae — closest relative of green plants 2. Brown algae — includes the largest seaweeds 3. Diatoms — examples of stramenopiles 4. Dinoflagellates — glass, two-part shells 5. Red algae — has no flagellated stages in life cycle (A) 1234 (B) 2345 (C) 1245 (D) 1235 (E) 2345

48. Which of the following is an example of an opportunistic pathogen that can cause a mycosis? (A) Claviceps pururea, which produces ergots on rye that can cause serious symptoms in humans if milled into flour (B) Ophiostoma ulmi, which causes Dutch elin disease (C) the ascomycetes that causes ringworm (D) Candida albicans, which causes vaginal yeast infections (E) the mold Penicillium, an ascomycete that is now grown in liquid culture to produce antibiotics

49. Which of the following hormones is produced by adipose cells that helps to control appetite? (A) Insulin (B) Neuropeptide Y (C) Glucagon (D) Leptin (E) Ghrelin

50. The high osmolarity of the renal medulla is maintained by all of the following except ________. (A) diffusion of salt from the ascending limb of the loop of Henle (B) active transport of salt from the upper region of the ascending limb (C) the spatial arrangement of juxtamedullary nephrons (D) diffusion of urea from the collecting duct (E) diffusion of salt from the descending limb of the loop of Henle .

51. When light strikes the pigment rhodopsin in a rod cell, retinal dissociates from opsin, initiating a signal-transduction pathway that ________. (A) depolarizes the neighboring bipolar cells and initiates an action potential in a ganglion cell (B) depolarizes the rod cell, causing it to release the neurotransmitter glutamate, which excites bipolar cells (C) hyperpolarizes the rod cell, reducing its release of glutamate, which excites some bipolarcells and inhibits others (D) hyperpolarizes the rod cell, reducing its release of glutamate, which excites amacrine cells but inhibits horizontal cells (E) converts cGMP to GMP, opening sodium channels and hyperpolarizing the membrane, causing the rhodopsin to become bleached