Major Source Of Xtracellular Cholesterol

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287. The major source of extracellular cholesterol for human tissues is a. Very-low-density lipoproteins (VLDLs)

b. Low-density lipoproteins (LDLs)

c. High-density lipoproteins (HDLs)

d. Albumin e. y-globulin

288. The synthesis of 3-hydroxy-3-methylglutaryl CoA can occur a. Only in mitochondria of all mammalian tissues b. Only in the cytosol of all mammalian tissues c. Only in the endoplasmic reticulum of all mammalian tissues d. In both the cytosol and mitochondria e. In lysosomes

289. When the liver is actively synthesizing fatty acids, a concomitant decrease in P oxidation of fatty acids is due to a. Inhibition of a translocation between cellular compartments b. Inhibition by an end product c. Activation of an enzyme d. Detergent effects e. Decreases in adipocyte lipolysis

290. A 4-year-old girl presents in the clinic with megaloblastic anemia and failure to thrive. Blood chemistries reveal orotic aciduria. Enzyme measurements of white blood cells reveal a deficiency of the pyrimidine biosynthesis enzyme orotate phosphoribosyltransferase and abnormally high activity of the enzyme aspartate transcarbamoylase. Which one of the following treatments will reverse all symptoms if carried out chronically?

a. Blood transfusion b. White blood cell transfusion c. Dietary supplements of phosphoribosylpyrophosphate (PRPP)

d. Oral thymidine e. Oral uridine

291. In most patients with gout as well as those with Lesch-Nyhan syndrome, purines are overproduced and overexcreted. Yet the hypoxanthine analogue allopurinol, which effectively treats gout, has no effect on the severe neurological symptoms of Lesch-Nyhan patients because it does not a. Decrease de novo purine synthesis b. Decrease de novo pyrimidine synthesis c. Diminish urate synthesis d. Increase phosphoribosylpyrophosphate (PRPP) levels e. Inhibit xanthine oxidase

292. Which of the following would rule out hyperuricemia in a patient?

a. Lesch-Nyhan syndrome b. Gout c. Xanthine oxidase hyperactivity d. Carbamoyl phosphate synthase deficiency e. Purine overproduction secondary to Von Gierke's disease

293. Which one of the following contributes nitrogen atoms to both purine and pyrimidine rings?




Carbamoyl phosphate


Carbon dioxide





294. Which statement best describes xanthine?

a. It is a direct precursor of guanine b. It covalently binds to allopurinol c. It is a substrate rather than a product of the enzyme xanthine oxidase d. It is oxidized to form uric acid e. It is oxidized to form hypoxanthine

295. Feedback inhibition of pyrimidine nucleotide synthesis can occur by which of the following means?

a. Increased activity of carbamoyl phosphate synthetase b. Increased activity of aspartate transcarbamoylase c. CTP allosteric effects d. UMP competitive inhibition e. TTP allosteric effects

296. Purine nucleotide biosynthesis can be inhibited by which of the following?

a. Guanosine triphosphate (GTP)

b. Uridine monophosphate (UMP)

c. Adenosine monophosphate (AMP)

d. Adenosine triphosphate (ATP)

e. Inosine diphosphate (IDP)

297. Which base derivative can serve as a precursor for the synthesis of two of the other base derivatives shown?

a. Cytidine triphosphate (CTP)

b. Uridine monophosphate (UMP)

c. Deoxythymidine monophosphate (dTMP)

d. Adenosine triphosphate (ATP)

e. Deoxyadenosine monophosphate (dAMP)

298. Which is the rate-controlling step of pyrimidine synthesis that exhibits allosteric inhibition by cytidine triphosphate (CTP)?

a. Aspartate transcarbamoylase b. Hypoxanthine-guanine phosphoribosyl transferase (HGPRT) 5-fluorouracil c. Thymidylate synthase d. Ribose-phosphate pyrophosphokinase e. Xanthine oxidase

299. Which of the following compounds is a required substrate for purine biosynthesis?

a. 5-methyl thymidine b. Ara C

c. Ribose phosphate d. 5-phosphoribosylpyrophosphate (PRPP)

e. 5-fluorouracil

300. Which of the following compounds is an analogue of hypoxanthine?

b. Allopurinol c. Ribose phosphate d. 5-phosphoribosylpyrophosphate (PRPP)

301. A pentose with a 5'-phosphate group, a 2'-hydroxyl group, and a 1'-pyrimidine group describes which of the following structures?

a. Cytosine b. Guanosine c. Thymidine d. Thymidylate e. Cytidylate

302. Which of the activated groups or units is most closely associated with uridine diphosphate (UDP)?

a. Electrons b. Phosphoryl c. Acyl d. Aldehyde e. Glucose

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  • carter
    Which of the following is the major source of extracellular cholesterol for human tissues?
    4 months ago

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