step1 Biochemistry 문제풀이
운영자 / 2005-09-27 16:12:37
1.How many molecules of carbon dioxide are generated when one molecule of acetyl CoA enters the Krebs Cycle?
A. zero; no CO2 is evolved
B. one equivalent
C. two equivalents
D. three equivalents
E. four equivalents
(C) is correct. Two equivalents of CO2 are evolved, completely combusting acetyl CoA to CO2 and H2.
2.A premature male infant is born in respiratory distress. In addition to mechanical ventilation until such times as the lungs become fully developed, what compound can be administered that will significantly decrease the morbidity and mortality of this situation?
A phosphatidylserine
B phosphatidylcholine
C phosphatidylethanolamine
D dipalmitoylphosphatidylcholine
E mucin
(D) is correct. This is the chemical composition of lung surfactant which functions to decrease surface tension and is produced by type II pneumocytes which are late to develop. Although (B) is used to synthesize surfactant, the other compounds do not play a role in lung physiology.
3.A 3-year-old boy presents to the emergency department after a seizure witnessed by his mother. Further questioning of the mother indicates that this is the first such episode of which she is aware. It appears the boy has failed to attain the expected developmental milestones and her description of his baseline is consistent with moderate to severe mental retardation. Physical exam shows some microephaly along with hypopigmented skin and hair. There is no family history of such a condition. Which enzyme is most likely mutated in this patient?
a tyrosinase
b phenylalanine hydroxylase
c phenylalanine reductase
d cystathione b synthase
e tyrosine hydroxylase
(B) is correct. This patient has phenylketonuria (type I) which is an excess of phenylalanine due to an inability to convert it to tyrosine. The majority of these patients have a mutation in this enzyme, however, mutations can be seen (although less often) in dihydropteridine reductase (type II) or any tetrahydrobiopterin synthesis pathway enzymes (type III).
Choice (A) is incorrect. This enzyme would be inhibited in this patient by the excess phenylalanine (thus the hypopigmentation) but it is not mutated.
Choice (C) is incorrect. Not a real enzyme.
Choice (D) is incorrect. Mutations in this enzyme cause homocystinuria and is associated with retardation, marfanoid habitus, optic lens dislocation, and osteoporosis.
Choice (E) is incorrect. An enzyme which also uses tetrahydrobiopterin but is involved in synthesis of neurotransmitters.
4.UV irradiation, particularly from the sun resulting in "sunburn", has been tightly associated with certain kinds of skin cancer. The origin of this cancer is traced to DNA mutations. Which of the following best describes the mechanism by which UV irradiation mutates DNA?
it produces dimers from cytosine and thymidine
it produces breaks in the sugar phosphate backbone
it produces deamination of cytosine to uridine
it produces base loss
it produces methylation
(A) is correct. These are the pyrimidine dimers often heard of with regard to UV irradiation. They are removed by excision repair.
Choice (B) is incorrect. This is seen with X-ray irradiation.
Choices (C) and (D) are incorrect. This is caused by heat exposure and some chemical toxins.
Choice (E) is incorrect. This is caused by chemical toxins.
5.After a number of episodes of severe sunburn during early childhood, a 4-year-old girl has developed several skin tumors on the hands, back of neck, and tip of nose. Other family members have no history of malignant skin tumors or melanoma. Which is the most likely diagnosis for this child?
Cockaynes syndrome
xeroderma pigmentosa
ataxia telangiectasia
Fanconi's anemia
Friederich's ataxia
(B) is correct. This is the only one of the above which would present at such an early age with skin tumors in that distribution.
Choice (A) is incorrect. This autosomal recessive disorder is associated with mental retardation, dwarfism, retinal degeneration in addition to skin hypersensitivity which can present with tumors later than xeroderma pigmentosa.
Choice (C) is incorrect. This autosomal recessive disorder is associated with profound ataxia presenting in the first decade of life and an increased risk of cancer. There is also an increased risk of some cancers for the heterozygotes. Site of mutation is ATM gene (a.t. mutated) which is a kinase.
Choice (D) is incorrect. This autosomal recessive disorder is associated with progressive pancytopenia and increased cancer risk. Patients are typically short with mental retardation and skeletal abnormalities.
Choice (E) is incorrect. This autosomal recessive disorder is associated with ataxia presenting before 25 years of age and cardiac defects. It is a triplet repeat disease and would therefore show anticipation.
6.AZT (3' azido 3' deoxythymidine) is used to treat HIV infection. The function of this drug is to inhibit
viral entry into cells
reverse transcriptase
thymidine synthetase
mRNA synthesis
ribonucleotide reductase
(B) is correct. The drug is a nucleoside analog. It is converted into the triphosphate and recognized as a correct nucleotide by the viral reverse transcriptase. Without a 3' OH group, further synthesis of DNA ceases. In addition, the azido group can react with the reverse transcriptase protein and inactivate it.
7. If the coding strand of a piece of DNA reads 5'-AAGTCCATCGACG-3', then what is the primary transcript RNA from that segment of DNA?
5'-AAGTCCATCGACG-3'
5'-AAGUCCAUCGACG-3'
5'-UUCAGGUAGCUGC-3'
5'-CGUCGAUGGACUU-3'
5'-GCAGCTACCTGAA-3'
(B) is correct. The coding strand of DNA is complementary to the template strand, which is used for transcription of RNA. Since the primary transcript will be complementary to the template strand, the primary transcript should be identical to the coding strand of DNA except uracil (U) replaces thymine (T) because it is RNA not DNA. Therefore, (B) is the correct answer.
8.Which of the following best describes the technique used to prenatally determine the deletion of a gene using a radioactive probe?
polymerase chain reaction
antisense technology
southern blot
restriction fragment length polymorphism
western blot
(C) is correct. Although PCR could also be used it would not require the use of a radioactive probe.
9.Which of the following best describes the technique used to prenatally determine the deletion of a gene using a radioactive probe?
polymerase chain reaction
antisense technology
southern blot
restriction fragment length polymorphism
western blot
(C) is correct. Although PCR could also be used it would not require the use of a radioactive probe.
10.The frequency of sickle cell anemia in the African-American population in the United States is 0.16%. What is the frequency of the carrier state in this population?
2%
4%
7.5%
15%
25%
(C) is correct. The formula for the Hardy-Weinberg law is 1=p2 + 2pq + q2. The frequency of the gene for sickle cell is the square root of the frequency of the homozygote or square root of 0.0016 = 0.04. The frequency of the normal allele (p) is 1 - q = 0.96. The frequency of the heterozygote is 2pq=0.0768 or 7.68% of this population can be expected to be carriers of the gene for sickle cell anemia.
11. A new drug has been discovered that inhibits the HMG CoA reductase enzymatic reaction by binding the active site. Which of the following kinetic changes would occur as a result of administration of this new drug?
An increase in the apparent Km
An increase in the Vmax
A decrease in the apparent Km
A decrease in the Vmax
No change in both the Km and the Vmax
(A) is correct. The new drug is behaving like a competitive inhibitor. Based on Michaelis-Menten kinetics, the apparent Km will increase, but the Vmax will remain the same. In other words, if enough substrate is added to the system, the inhibition will be reversed
12.The pentose phosphate pathway is an alternative way for the cell to metabolize glucose; however, its level of activity is closely regulated according to the needs of each individual cell. Of the many possible cellular functions of this pathway, which of the following is most important in a mature red blood cell?
to produce energy in the form of ATP
to produce NADPH
to produce CO2
to produce NADH
to produce pentose sugars for DNA synthesis
(B) is correct. NADPH reducing equivalents are necessary to keep a high cellular concentration of reduced glutathione.
Choice (A) is incorrect. This pathway is not physiologically used as a major source of energy.
Choice (C) is incorrect. Although CO2 is produced in the pentose phosphate pathway, the biological purpose of the pathway is to produce NADPH.
Choice (D) is incorrect. NADH is not formed by this pathway, NADPH is.
Choice (E) is incorrect. Altough pentoses can be formed, as the name implies, DNA synthesis is not required of a mature red blood cell.
13. Parents of a 5-month-old boy are concerned because he does not focus on objects placed in front of him. Past medical history reveals that the boy had a seizure 2 months ago that was attributed to fevers. Physical examination demonstrates a cherry red spot in the macula, and some weakness in muscle tone and grasp. Heart, lungs, liver, and spleen are all within normal limits. Which of the following is the most likely diagnosis?
Tay-Sachs disease
Niemann-Pick disease
Fabry's disease
Gaucher's disease
Krabbe's disease
(A) is the correct answer. The boy demonstrates the classic symptoms of Tay-Sachs disease, an autosomal recessive disease characterized by a deficiency in hexosaminidase A, leading to an accumulation of ganglioside GM2. Most cases result in early death, typically before age 3. Niemann-Pick disease also has a cherry-red spot; however, it is also typified by hepatosplenomegaly, which this child did not have. Fabry's disease is an X-linked disorder of a-galactosidase deficiency resulting in kidney and heart failure. Gaucher's disease is due to a deficiency in glucosidase resulting in hepatosplenomegaly, mental retardation, and osteoporosis. Krabbe's disease is a disorder in b-galactosidase that results in mental retardation, blindness, deafness, and early death. Metachromatic leukodystrophy is a deficiency in arylsulfatase A, which causes mental retardation, demyelination leading to paralysis and dementia, and a yellowish-brown appearance to neurons stained with cresyl violet (metachromasia).
14. A 2-year-old boy presents with mental retardation. Physical examination reveals dislocation of the lens, normal heart and breath sounds, and normal-sized liver and spleen. Urinalysis reveals elevated homocysteine. Which of the following enzymes is most likely to be deficient?
Homogentisate oxidase
Branched chain ketoacid dehydrogenase
Cystathionine synthetase
Tyrosinase
Phenylalanine hydroxylase
(C) is correct. The patient is presenting with clinical symptoms of homocystinuria, confirmed by elevated urinary homocysteine levels. The enzyme that is deficent is cystathionine synthetase. Homogentisate oxidase is deficient in alcaptonuria, characterized by dark urine. Branched chain ketoacid dehydrogenase is deficient in Maple-Syrup Urine Disease, characterized by mental retardation, severe neurological problems, sweet smelling urine, and early death. Tyrosinase is deficient in albinism and presents with complete apigmentation. Phenylalanine hydroxylase is deficient in PKU, characterized by blond hair, blue eyes, mental retardation and elevated phenylketones in the urine.
15. Clotting malfunction is most related to a deficiency of
vitamin A
vitamin B12
vitamin C
vitamin K
vitamin E
(D) is correct. Clotting malfunction can be related to vitamin K deficiency and results in a prothrombin deficiency.
16.Which of the following enzymes is inhibited by methotrexate?
dihydrofolate reductase
thymidine synthetase
ribonucleotide reductase
carbamoyl phosphate synthetase
AMP synthetase
(A) is correct. Methotrexate is a folate analog that inhibits dihydrofolate reductase.
17.What is the enzyme defect seen in young boys with an X-linked form of mental retardation associated with gout?
phosphoribosylpyrophosphate synthetase
xanthine oxidase
hypoxanthine phosphoribosyl transferase
adenosine deaminase
purine nucleotide phosphorylase
(C) is correct. This is a patient with Lesch-Nyhan syndrome. The joint and back pain are gout and uric acid lithiasis respectively.
Choice (A) is incorrect. No deficiency is described for this enzyme.
Choice (B) is incorrect. This is the enzyme inhibited by the drug allopurinol.
Choice (D) is incorrect. This deficiency causes severe combined immunodeficiency disease.
Choice (E) is incorrect. This deficiency causes a T cell mediated immunodeficiency.
18. Which of the following is true regarding autosomal dominant inheritance?
all of the offspring of an affected parent will be affected
males will show the trait more often than females
the protein product of the gene shows a gain in function
non-affected parents can have affected offspring
the trail can skip generations
(C) is correct. In autosomal dominant inheritance, approximately half of the offspring of an affected parent will show the trait. Males and females will be equally affected, generations are not skipped, non-affected parents have non-affected offspring, and the protein product shows a gain in function.
19.A 16-year-old boy with a lanky body habitus presents with a routine otitis media, but on physical exam he is found to have atrophic testicles and no hint of developing secondary sex characteristics. By his own description he does not do well in school. Blood tests show very high FSH levels with barely detectable levels of testosterone. What is the most likely karyotype of this patient?
47, XY +21
47, XXY
45, X
47, XYY
46, XX
(B) is correct. This is the classic description of a Klinefelters patient.
Choice (A) is incorrect. This patient does not have Down syndrome despite the poor school performance.
Choice (C) is incorrect. This is a short phenotypically female patient without sexual maturation.
Choice (D) is incorrect. XYY syndrome is characterized by abnormally tall stature and possibly a tendancy towards aggression.
Choice (E) is incorrect. Even if this boy did not have a chromosomal abnormality he is not a normal female.
20.Enzymes that exhibit allosteric regulation require a level of protein organization which may not be found in all polypeptides. Which of the following is the best description of that order of protein structure?
primary structure
secondary structure
tertiary structure
quaternary structure
amino-acid sequence
(D) is correct. The best answer is quaternary structure because it is the interaction of multiple subunits that gives allosteric enzymes their unique regulatory attributes. The definition of quaternary structure is the relationship between different polypeptides in a multi-subunit protein. The amino acid sequence referred to in distractor (E) is the same as primary structure.
