23. Which structure shown in the figure above is a ketohexose? ①② ③④
(A) ①
(B) ②
(C) ③
(D) ④
(E) None of the above

24. Which of the following would NOT decrease activity of the citric acid cycle overall? (A) High concentration of citrate (B) High concentration of NADH (C) High concentration of Ca2+ (D) High concentration of ATP (E) All of the above

25. Which of the following accurately ranks lipoproteins from highest to lowest density? (A) chylomicrons ＞ VLDL ＞ IDL ＞ LDL ＞ HDL (B) chylomicrons ＞ HDL ＞ LDL ＞ IDL ＞ VLDL (C) HDL ＞ IDL ＞ LDL ＞ VLDL ＞ chylomicrons (D) HDL ＞ LDL ＞ IDL ＞ VLDL ＞ chylomicrons (E) HDL ＞ VLDL ＞ LDL ＞ IDL ＞ chylomicrons

26. In a protein, the most conformationally restricted amino acid is______; the least conformationally restricted is ______. (A) Ala, Pro (B) Ile, Ala (C) Met, Cys (D) Pro, Gly (E) Trp, Gly

27. In a hepatocyte under high energy conditions, PFK-1 activity is significantly reduced. A simultaneous increase in which of the following would most effectively overcome this inhibition and reactivate glycolysis? (A) Citrate (B) Fructose-2,6-bisphosphate (C) Glucose-6-phosphate (D) ATP (E) Fatty acids

28. A man taking multiple medications develops drug toxicity. Laboratory analysis reveals impaired Phase I metabolism. Which of the following reactions is most likely deficient in this patient? (A) Hydroxylation by cytochrome P450 (B) Glucuronidation by UDP-glucuronyltransferase (C) Sulfation by sulfotransferase (D) Glutathione conjugation by glutathione S-transferase (E) Acetylation by N-acetyltransferase

29. A researcher incubates hepatocytes with 15NH4Cl and observes that 15N appears first in glutamate, then subsequently in aspartate. Which of the following best explains this sequential labeling? (A) 15NH4+ → glutamine (by glutamine synthetase) → glutamate → aspartate (by transamination) (B)  15NH4+→ glutamate (by glutamate dehydrogenase) → aspartate (by transamination with oxaloacetate) (C) 15NH4+→ carbamoyl phosphate (by CPS I) → glutamate → aspartate (via urea cycle intermediates) (D)  15NH4+ → glutamate (by glutamate dehydrogenase using succinate) → aspartate (by transamination with fumarate) (E)  15NH4+→ glutamate (by glutamine synthetase) → aspartate (by reductive amination of oxaloacetate)

30. Which enzyme is responsible for the synthesis of prostaglandins and thromboxanes from arachidonic acid? (A) Lipoxygenase (B) Cyclooxygenase (C) Desaturase (D) Elongase (E) Peroxidase

31. During a stressful event, the body produces both immediate and long-term physiological responses. Which of the following best describes how the hypothalamus and adrenal glands work together in this stress response? (A) The hypothalamus releases corticotropin-releasing hormone (CRH) directly into the bloodstream to stimulate the adrenal medulla to secrete glucocorticoids. (B) The adrenal medulla secretes glucocorticoids and mineralocorticoids, which produce rapid "fight-or-flight" responses. (C) The hypothalamus signals the pituitary gland to release adrenocorticotropic hormone (ACTH), which stimulates the adrenal cortex to secrete glucocorticoids during long-term stress. (D) Mineralocorticoids released from the adrenal medulla primarily increase heart rate during acute stress. (E) During the short-term stress response, the anterior pituitary releases epinephrine directly into the bloodstream to bypass the slower neural pathways of the sympathetic nervous system.

32. During DNA replication of an eukaryotic cell, a guanine (G) is mistakenly paired with a thymine (T) and escapes the initial proofreading by DNA polymerase. This G-T pair becomes fully incorporated into the double helix. Which of the following best describes how the cell corrects this mismatch/error? (A) The cell utilizes DNA photolyase to break the hydrogen bonds between the mismatched bases and flip them into the correct orientation. (B) The cell utilizes specific endonucleases to recognize the helical distortion and identify the newly synthesized strand to cut out the segment containing the incorrect nucleotide. (C) The cell utilizes RNA primase to recognize the mismatched sites, where it initiates reverse transcription to replace the DNA segment with a temporary RNA patch. (D) The cell utilizes DNA ligase to identify the incorrect hydrogen bonding site and uses its 5' to 3' exonuclease activity to chew back the faulty nucleotide. (E) The cell utilizes a multi-protein complex to scan the DNA for bulky lesions and remove a 12-30 nucleotide oligomer of the damaged strand.

33. Long-term potentiation (LTP) represents a fundamental cellular mechanism underlying learning and memory. Which of the following best describes a critical requirement for the induction of early-phase LTP? (A) Sustained activation of NMDA receptors leading to Ca2+ influx, which directly promotes rapid insertion of AMPA receptors into the postsynaptic membrane. (B) Ca2+ influx through NMDA receptors following postsynaptic depolarization, which triggers the translocation of additional AMPA receptors to the postsynaptic membrane via calcium/calmodulin-dependent protein kinase II (CaMKII) signaling. (C) Increased presynaptic glutamate release caused by enhanced vesicle docking, which by itself is sufficient to maintain persistent synaptic potentiation. (D) Activation of postsynaptic glycine receptor chloride channels, which leads to the depolarization that relieves the Mg2+ block of NMDA receptors. (E) Immediate activation of cAMP response element-binding protein (CREB)-dependent gene transcription in the absence of Ca2+ influx, which primes neurons for later potentiation.

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