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USMLE Step 1 Biochemistry

Subjects : health-sciences, usmle-step-1

Instructions:

Answer 50 questions in 15 minutes.
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Match each statement with the correct term.
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This is a study tool. The 3 wrong answers for each question are randomly chosen from answers to other questions. So, you might find at times the answers obvious, but you will see it re-enforces your understanding as you take the test each time.

1. What is the pathogenesis of fragile x syndrome?
Dermatitis - glossitis - diarrhea
In the charging reaction! for this reason aminoacyl tRNA synthetase double checks
Asp and glu; negatively
A trinucleotide repeat disorder results in faulty methylation and thus expression of the FMR1 gene on the X chromosome (x linked recessive mode of inheritance)

2. What does severe deficiency of B3 lead to? What are the causes?
2/3; LCAT (lecithin cholesterol acyltransferase)
Codons may encode same amino acid since there are only ~20 aa
Where there is insulin; low protein kinase A means dePhosphorylation of the complex Which results in increased PFK2 Which results in more fructose 2 -6 bp and thus more PFK1!!
Pellagra (dermatitis - diarrhea - dementia); INH use - B6 deficiency - Hartnup disease - malignant carcinoid syndrome - and a corn based diet (lacks trp and niacin in corn cant be absorbed)

3. What is the net production of glycolysis from one glucose?
Decreased would healing; microcytic anemia (d/t decreased iron [not absorbable]; macrocytic anemia (d/t Fh4 not reduced)
Acetyl CoA carboxylase
2 NADH - 2 ATP - 2 pyruvate - 2H+ - 2H20
Glucosekinase to glucose 6 P

4. Insulin receptor is not cAMP second receptor but rater tyrosine kinase...
Which dimerizes and activates a protein phosphatase Which de phosphorylates glycogen phosphorylase
Lens and neural tissue; converts galactose to osmotically active galactitol
activation of PFK1
Its the ribose; its made from PRPP synthetase using ribose 5 P from the HMP shunt

5. other than bone resorption to release Ca and phosphate - What is the ultimate point of increasing the serum Ca2+ and phosphate?
When the disease manifestation depends on from who the gene deletion was inherited from d/t methylation that occurs; Prader Willi and AngelMann
Glucose 6 phosphate dehydrogenase
To increase serum Ca and phosphate for bone deposition
Pompes

6. What toxins prevent elongation in translation in humans (by binding 40s)?
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens
When water is pulled in; polyols (sugar alcohols); cataracts - retinopathy - peripheral neuropathy all seen in chronic hyperglycemia in diabetes
Shiga toxin and ricin (protein in castor beans)

7. What aa does AUG mRNA code for in eukaryotes? prokaryotes?
(cofactor for 100+ enzymes!) needed for zinc fingers transcription motif - collagen synthesis - alcohol metabolism
2 oxidative and nonoxidative; cytoplasm (both); none (none produced either - you are simply switching carriers of energy- from glucose 6 phosphate to NADPH)
Methionine; f Met (formyl methyl methionine)
Alcoholism - RA - inflammatory disease - chronic diarrhea

8. Why do mitochondrial diseases exhibit variable expression?
AR
Heteroplasmy
GALT
Alanine (through pyruvate) - aspartate (through OXA) - glutamate (alpha ketoglutarase); TCA

9. What does a def in zinc result in?
Hemolytic anemia with a blood smear with heinz bodies and bite cells (phagocytic removal of heinz bodies)
Delayed wound healing - hypogonadism - decreased adult hair - dysgeusia - anosmia - and alcoholic cirrhosis - perioral rash
Galactose
Night blindness - dry skin

10. What are the three types of point mutations?
Silent - misssense - nonsense
homocysteine transferase
Phenylalanine hydroxylase; tetrahydrobiopterin factor
Arg and his

11. Which RNA polymerase synthesizes tRNAs and snRPs?
III
show disease
Reserve supply of glucose in the fasting state - liver gives to the blood and skeletal muscle uses for itself
To amplify a specific desired fragment of DNA

12. What is the deficient enzyme in Krabbes disease? accumulated Substrate?
Flouroquinolones
Galactocerebrosidase; Galactocerebroside
Tryptophan; B6
Fructose bisphosphatase 2 and Phosphofructokinase 2; phosphorylation by protein kinase A; inuslin/glucagon ratio

13. What are the three mc autosomal trisomies in decreasing order?
Downs (21) - Edwards (18) - Pataus (13); also in increasing severity
Aldolase B; Fructose 1 phosphate accumulates Which uses up Phosphate and the result is inhibition of gluconeogenesis without phosphate; hypoglycemia - jaundice - cirrhosis - vomitting
dimers
Some viruses

14. What enzyme is deficient in galactokinase deficiency? What are the symptoms?
Following gastroenteritis and kwashiokor (protein deficiency)
Shiga toxin and ricin (protein in castor beans)
Because that is Where the products will be consumed; acetyl coA; TCA cycle of ketone body production
Galactokinase; galactose appears in blood and urine - sometimes infantile cataracts may present as failure to track objects or lack of a social smile

15. What are permanent cells? Which cells are permanent?
NADH - Acetyl CoA - ATP; NAD+ - ADP and Ca2+
NAD+; through ETC you produce NAD+; through production of Lactate from pyruvate you create lactate
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs
It induces its action!

16. What does arsenic do? How is this important?
Arginine
Mitochondrial inheritance
Uracil; thymine
Inhibits lipoic acid; lipoic acid is a cofactor for pyruvate dehydrogenase

17. What are the findings in maple syrup disease? What should be tried to be given as treatment?
Chromosomal imbalance (downs for ex)
No; increased mutation rate!
CNS defects - mental retardation - death - feeding difficulties - vomiting - hypoglycemia; thiamine
A piece of glass or silicon chip with thousands of grids with different nucleic acid sequences and DNA or RNA probes are hybridized to the chip and a scanner detects the complementary binding; used to study a persons genome and to detect snps (single

18. AD disease...
Vit K; neonates have sterile intestine and can not synthesize vit K
tyrosine hydroxylase
usually present late
Glucose 6 phosphate dehydrogenase

19. What is the first step in fatty acid synthesis? Where does it occur? Where does triacylglycerol synthesis occur?
Osmotic diarrhea - bloating - cramps
Citrate shuttle in the inner mitochondrial membrane transports acetyl coA into the cytoplasm; liver; liver and adipose tissue
Microcephaly - mental retardation - high pitched mewing/crying - epicanthal folds - cardiac abnormalities; microdeletion on short arm of chrom 5
It induces its action!

20. What does the Cre - lox system allow in Gene expression modifications?
Glucosekinase to glucose 6 P
Vitamin B1 (thiamine) - Vitamin B2 (riboflacin) - Vitamin B3 (niacin) - Vitamin B5 (pantothenate) - Vitamin B6 (pyridoxine) - Vitamin B7 (Biotin) - Vitamin B12 (cobalamin) - folate - Vit. C
Exons
Can inducibly manipulate genes at specific developmental points

21. What drug blocks acetaldehyde dehyrogenase? What are its side effects?
ATP
Disulfiram (antabuse); acetaldehyde accumulate contributing to hangover
BM; laminins; most flexible; different aa composition
glycogenolysis

22. What causes an excess of B3? What are the symptoms?
Cells that are stable go in and out of the G1 to G0 Which is the stable/quiescent (can go into replication but dont) type of cells; cells that stay in G0 are permanent and can not enter G1
Pellagra
Niacin treatment for hyperlipidemia; facial flushing
Cheilosis and corneal vascularization

23. Homocysteine gets turned into methionine by...
Actin/myosin - MT - intermediate filaments
2-3 days after birth because maternal enzyme during fetal like might still be there
Prevents neural tube defects in 1st trimester
homocysteine transferase

24. What type of aa is phenylalanine? What implications does this have on the presentation of phenylketonuria?
Orotic acid in blood and urine (from pyrimidine synthesis) - decreased BUN - and symptoms of hyperammonemia
Aromatic; musty body odor
Hurlers Syndromes;Iduronate sulfatase deficiency; alpha L iduronidase def; heparan sulfate - dermatan sulfate
liver and muscle

25. What does 6 mercaptopurine do?
Microvilli - muscle contraction - cytokinesis - adherens junctions
Either salvage by HGPRT and PRPP or converstion to xanthine and then to uric acid by xanthine oxidase
6MP blocks de novo purine synthesis
Increased NADH causes lactate production and pyruvate dehydrogenase def causes increased lactate (d/t thiamine def that often occurs with alcoholics)

26. In What syndrome are their immotile cilia? why?

27. What is the presentation of G6PD deficiency?
Helps make Vit K and antioxidant
liver and muscle
Hemolytic anemia with a blood smear with heinz bodies and bite cells (phagocytic removal of heinz bodies)
NADH - Acetyl CoA - ATP; NAD+ - ADP and Ca2+

28. What is the exception to degeneracy in the genetic code?
Thinned cornea from lack of connective tissue so see underlying choroidal lens
Tay Sachs - Niemann Picks - Gauchers
high Vmax
Methionine is only encoded by AUG

29. What is the importance of folic acid?
Endonucleases are specific and break specific linking phosphodiester bonds - exonucleases are not sequence specific and just remove the nucleoside
By turning it into fructose - but sorbitol first; glucose to sorbitol via aldose reductase and sorbitol to fructose sorbital dehydrogenase
Prevents neural tube defects in 1st trimester
Encode enzymes for oxphos mit reactions

30. Where is type III collagen found?
Riboflavin; dairy and meat
Reticulin - skin - blood vessels - uterus - fetal tissue - early phase of granulation tissue
Coarse facial features - psychomotor retardation - clouded corneas - restricted joint movement - high plasma levels of lysosomal enzymes
Galactokinase; galactose appears in blood and urine - sometimes infantile cataracts may present as failure to track objects or lack of a social smile

31. Which end carries the triphosphate? Which end of DNA makes the hydoxyl attack?

32. _______ is ubiquitous
In hepatocytes; B6
Collections of abnormal mit collecting under sarcolemmal membranes and results in distortion of myofibrils
7 dehydrocholesterol
hexokinase (glucokinase only found in liver and beta cells of pancreas)

33. High blood levels of fructose - galactose - glucose can result in conversion to osmotically active alcohol forms by...
Protein Kinase A inhibits pyruvate kinase also ATP and alanine have negative effects on pyruvate kinase
Hurlers Syndromes;Iduronate sulfatase deficiency; alpha L iduronidase def; heparan sulfate - dermatan sulfate
aldose reductase
Depression - progressive dementia - choreiform movements; between the ages of 20 and 50

34. What step in pyrimidine synthesis requires aspartate?
Carbamoyl phosphate to orotic acid
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -
high cholesterol
Increased TG synthesis from increased Glycerol 3 P and abetalipoproteinemia

35. What is the main hormonal regulation of glycogen?
Glucagon - insulin and epinephrine
Hydroxurea; deoxygenates ribose
DHF
Exons

36. What are the findings of homocystinuria?
Increased CPK and muscle biopsy
Increased homocysteine in the urine - mental retardation - osteoporosis - marfanoid habitus - and atherosclerosis
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis
Formation of the alpha chains out of glycine - lysine - proline in the RER to make preprocollagen; hydroxylation of the proline and lysine residues in the RER; glycosylation of the lysine residues to make procollagen Which is triple helix of alpha ch

37. After the PCR reaction - How do you actually visualize the DNA?
Do agarose gel eletrophoresis to separate the different sizes of the PCR products (sizes id the the fragment)
Flouroquinolones
1 gene has greater than 1 phenotypic effect; locus heterogeneity (many different mutations can cause the same phenotype); PKU; albinism - deafness - Marfanoid habitus (Marfans - MEN 2B - homocystinuria)
Debranching enzyme; alpha 1 - 4 glucosidase; no!; Pompes disease

38. Vit A is ________ - causes cardiac problems and cleft palate in newborns
Glutamate and aspartate; CO2 provides an ketone
Galactokinase; galactose appears in blood and urine - sometimes infantile cataracts may present as failure to track objects or lack of a social smile
Phenylaline
teratogenic

39. What is deficient in someone with hereditary hypoketotic hypoglycemia?
Osmotic diarrhea - bloating - cramps
Carnitine shuttle
Increased CPK and muscle biopsy
Adrenal cells (steroid synthesis) - hepatocytes (p450 detox)

40. What is palmitate? saturated or unsaturated? What does that meat?
A 16C FA! the product in FA synthesis; saturated; no double bonds
Glycogen phosphorylase
Blood group types
RER in nucleus (synthesize NTs)

41. Prokaryotes have What ribosomal units?
Bilateral acoustic schwannomas - juvenile cataracts
Fe2+; Fe3+
30S + 50S= 70S
Fat malabsorption (CF) and abetalipoproteinemia (damage takes up all antioxidants)

42. What does a deficiency in homogentistic acid oxidase result in? What is this also known as?
Kartaganer's aka ciliar dyskinesia; lack of dynein arm ATPase
Alkaptonuria; ochronosis
Preprocollagen - procollagen - tropocollagen - collagen
Type I collagen; AD

43. What does degenerate/redundant mean in terms of the genetic code?
Cri du Chat
Arylsulfatase A; cerebroside sulfate
Codons may encode same amino acid since there are only ~20 aa
Decrease intake of fructose and sucrose (fructose and glucose)

44. What are the symptoms of williams syndrome? What is the pathogenesis?
Acetyl coA carboxylase converting Acetyl coA into malonyl coA; increased AMP - glucagon - epinephrine - palmitate; citrate
AST (mit damage) - and GGT (d/t SER hyperplasia d/t induction of p450 generation)
Distinctive elfin faces - mental retardation - hypercalcemia d/t sensitivity to vit D - well developed verbal skills and extreme friendliness - CV problems
Positive on phosphofructokinase 1 and negative on fructose 1 -6 bisphosphatase

45. Where is glycogen mainly found?
glycogenolysis
Debranching enzyme; alpha 1 - 4 glucosidase; no!; Pompes disease
Liver mit and liver cytosol
Liver and skeletal muscle

46. What are two reasons why pyruvate would be pushed to lactate in an alcoholic?
Defective neutral amino acid transporter on renal and intestinal cells; AR
Exons
5' capping (with 7 methylguanosine) - 3' adenylation - splicing of introns; nucleus
Increased NADH causes lactate production and pyruvate dehydrogenase def causes increased lactate (d/t thiamine def that often occurs with alcoholics)

47. Only the tyrosinase one is...
Acetazolamide to alkalinize the urine
AR
Collagen; fibroblasts; 4; ECM
Pyruvate to lactate (results in lactic acidosis and decreased pyruvate for oxaloacetate for gluconeogenesis) - increased production of glycerol 3 phosphate from DHAP for TG synthesis; and increased betahydroxybutyrate ketoacid production from increas

48. Achondroplasia is associated with advanced ________ age
paternal
only yield acetyl coA
Von gierkes
DiGeorge and Velocardiofacial; deletion on chromosome 22q11 results in aberrant dev of 3rd and 4th branchial pouches; digeorge has thymic and parathyroid prob - velocardiofacial has palate and facial instead

49. What is Maternal PKU?

50. How many RNA polymerases do prokaryotes have? What drug blocks prokaryotic RNA polymerase only? What is it used for?
1 for all three classes of RNA; Rifampin; TB
Kidney and intestinal epithelium
Glycolysis - fatty acid synthesis - HMP shunt - protein synthesis (RER) - steroid synthesis (SER) - glycogenolysis - glycogenesis
Aminoglycosides and tetracyclines