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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 difference between rickets and osteomalacia?
homocysteine transferase
Osteomalacia is vit D def seen in adults and causes bowed legs and results in increased fractures
Whether or not the patient is also hyperammonemia (if yes - then OTC def)
be anorexic

2. What liver enzymes are increased in alcoholism? why?
Ret
In the reproductive tract and in the resp tract; infertility - bronchiectasis and recurrent sinusitis - associated with situs inversus
AST (mit damage) - and GGT (d/t SER hyperplasia d/t induction of p450 generation)
Mcardles disease

3. How is urea measured
Tryptophan; niacin
As BUN (blood urea nitrogen)
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
Cartilage (including hyaline) - vitreous body - nucleus pulposus

4. What does thymine have on it?
Both add phosphate; phosphorylase uses no ATP and inorganic phosphate
Acetyl coA and NADH inhibit pyruvate dehyrogenase
SAM
A methyl

5. Where is anaerobic glycolysis used often?
Arylsulfatase A; cerebroside sulfate
By turning it into fructose - but sorbitol first; glucose to sorbitol via aldose reductase and sorbitol to fructose sorbital dehydrogenase
Mebendazole and thiabendazole
RBCs - renal medulla - leukocytes - lens - testes - cornea

6. What aa becomes essential with homocystinuria? phenylalaninuria?
Bacteria produce lactate acid and gases from lactose and produce acidic diarrhea and bloating
Tyrosinase deficiency; defective tyrosine transporters; lack of migration of neural crest cells; skin cancer
Cysteine; tyrosine
Lys and arg

7. What type of aa is phenylalanine? What implications does this have on the presentation of phenylketonuria?
Pataus
Triacylglycerols in adipose tissue broken down into glycerol and free fatty acids; growth hormone and epinephrine
Aromatic; musty body odor
Stored ATP - creatine phosphate - anaerobic glycolysis; as distances increase - ATP is obtained from additional resources; above plus ox phos

8. What are the three types of point mutations?
Thymidylate synthase; 5- Flourouracil
Citrate is positive on it - AMP and fructose 2 -6 bisphosphate is negative on it; gluconeogenesis
Lactose (glucose and galactose)
Silent - misssense - nonsense

9. What is 'charging' of tRNA? What enzyme regulates this? What does this reaction require? in What process is this?
Von gierkes
No; increased mutation rate!
glucagon - insulin and epinephrine
When the amino acid is covalently binded to its corresponding tRNA; Aminoacyl tRNA synthetase; ATP; translation

10. What dictates whether a cell will regenerate or repair after inflammation?
Phosphoenolpyruvate; pyruvate kinase; fructose 1 - 6 BP; ATP - alanine
1) if it is stable/labile or permanent 2) intact BM and 3) intact ECM
3 NADH - 1 FADH2 - 2 CO2 - 1 GTP = 12 ATP/acetyl CoA
In the reproductive tract and in the resp tract; infertility - bronchiectasis and recurrent sinusitis - associated with situs inversus

11. What does the passage of electrons in the ETC result in?
Synthesis in liver and then hydrolysis of chylomicrons and VLDLs by capillary lipoprotein lipase in peripheral tissue; adipose tissue by lipases
III
Increased TG synthesis from increased Glycerol 3 P and abetalipoproteinemia
Formation of a proton gradient that is then used to create ATP through ATP synthase

12. Which vesicular trafficking protein takes vesicles from ER to the Golgi (anterograde)?
Disulfiram (antabuse); acetaldehyde accumulate contributing to hangover
All and Alzheimers
Lime and CaCO3 to displace the niacin
COP II

13. What drug blocks ribonucleotide reductase? What does this enzyme do?
Glucose 6 phosphate dehydrogenase
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables
Ascites from liver damage
Hydroxurea; deoxygenates ribose

14. What aa does AUG mRNA code for in eukaryotes? prokaryotes?
1. denature by heating 2. during cooling anneal with DNA primers 3. elongation - add heat stable DNA polymerase 4. repeat
Methionine; f Met (formyl methyl methionine)
glutton
Marfans - homocystinuria - MEN 2B

15. _______ is ubiquitous
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
Robertsonian translocations
hexokinase (glucokinase only found in liver and beta cells of pancreas)
Cystathionine (which then goes to cysteine) via cystathionine reductase; B6

16. What is the rate determining enzyme in urea cycle?
Carbomyl phosphate synthetase I turns N acetyl glutamate into carbomyl phosphate and then ornithine transcarbomyalse turns carbomyl phosphate into citrulline (using ornithine)
4 -4 -9
Carbomyl phosphate synthetase I
Indirect inhibtion of the Na+/Ca+2 exchange resulting in increased intracellular calcium Which increases cardiac contractility

17. What are the two most common aminotransferase and What amino acids do they turn to What glycolytic intermediates?
P53 and Rb; p53 activates BAX Which inhibits BCL2 When there is DNA damage - Rb phosphorylation allows progression into S phase
Alanine aminotransferase (ALT) and aspartate aminotransferase (AST); alanine to pyruvate and aspartate to oxaxloacetate
D2 (ergocalciferol) - D3 (cholecalciferol) - D3 (cholecalciferol)
Vitamin C; facilitates iron absorption by making sure its in its reduced state (Fe2+) - hydroxylation for lysine and proline Which is necessary for crosslinking; necessary for dopamine Beta hydroxylase for conversion of dopamine to NE; keeps FH4 in i

18. Where is type IV collagen found? to What protein is bound? What is special about this type? How do the different types of collagen differ?
Karyotyping
Dry is just the symmetrical neuropathy with both motor and sensory symptoms; wet includes both the neuropathy and cardiac involvement of high output failure with peripheral edema - tachycardia - cardiomegaly
Mit encephalopathy - lactic acidosis - stroke like episodes
BM; laminins; most flexible; different aa composition

19. What is folic acid? What is its function?
Carbidopa
(cofactor for 100+ enzymes!) needed for zinc fingers transcription motif - collagen synthesis - alcohol metabolism
Smoking (FR takes up all the antioxidants); diets lacking fruit
Water soluble vitamin stored in liver; converted to THF by dihydrofolate reductase and is a coenzyme in 1 methyl transfers - impt for DNA synth

20. What is the mode of inheritance for the multiple endocrine neoplasias?
AD
Glycerol goes to liver for gluconeogenesis; free fatty acids bind to albumin in blood and travel to cells to be used as for energy
NADH - NADPH - FADH2
Milder form of type 1 with normal blood lactate levels; debranching enzyme (alpha -1 -6 glucosidase)

21. What are the key intermediates in the TCA?
Citrate - Isocitrate - alpha ketoglutarate - Succinyl CoA - Succinate - Fumarate - Malate - Oxaloacetate
= q
Schwann cells - lens - retina - kidneys because they only have aldose reductase and no sorbital dehydrogenase
Ehler Danlos; type III collagen def; joint dislocation - berry aneurysm - ecchymoses - organ rupture

22. What is the most common cause of Osteogenesis Imperfecta? What is the inheritance pattern?
Starts before age 5 with with pelvic girdle weakness with use of Gowers maneuver to stand up - it then progresses superiorly with muscle weakness (type 1 and 2!); waddling duck gait; hyporeflexia; pseudohypertrophy of calf muscles d/t fibrofatty repl
Bilateral acoustic schwannomas - juvenile cataracts
Back up of pyruvate and alanine and thus lactic acidosis occurs; can be d/t alcoholism (vit B1 def)
Type I collagen; AD

23. What two def. most commonly cause phenylketonuria?
Codons may encode same amino acid since there are only ~20 aa
Uses HMG coA reductase
Dark connective tissue - brown pigmented sclera - debilitating athralgias; homogentistic acid builds up and can be toxic to cartilage
Phenylalanine hydroxylase or tetrahydrobiopterin factor

24. What Abs block the binding between aminoacyl tRNA and rRNA by binding the 30S ribosome?
When the disease manifestation depends on from who the gene deletion was inherited from d/t methylation that occurs; Prader Willi and AngelMann
Aminoglycosides and tetracyclines
Increased homocysteine in the urine - mental retardation - osteoporosis - marfanoid habitus - and atherosclerosis
Helps make Vit K and antioxidant

25. What is the deficient enzyme in Fabry's disease? accumulated Substrate?
Alpha galactosidase A; ceramide trihexoside
Fat soluble; either endogenous production or exogenous; D2 ergocalciferol is ingested from plants and D3 cholecalciferol is formed in sun exposed skin (and can be consumed in fortified milk)
5' end; 3' end
hexokinase - glucokinase (its a glutton)

26. Where are ketone bodies made? What is the rate limiting enzyme? What are they made from?
Tyrosinase deficiency; defective tyrosine transporters; lack of migration of neural crest cells; skin cancer
Because cells of mononuclear phagocytic are esp rich in lysosomes
A 16C FA! the product in FA synthesis; saturated; no double bonds
Mitochondria in liver; HmG CoA synthetase; amino acids and beta oxidation of FA

27. What disease is associated with chromosome 13?
Tyrosinase
Carbamoyl phosphate to orotic acid
HnRNA; mRNA
Pataus

28. What is the effect of the Cardiac glycosides?
both liver and muscle (unlike glucagon)
long saturated fatty acids
AD
Indirect inhibtion of the Na+/Ca+2 exchange resulting in increased intracellular calcium Which increases cardiac contractility

29. CFTR channel ______ Cl into sweat - secretes NaCl in luminal secretions
reabsorbs
cell cycle events
HMG CoA reductase
Fructose 1 -6 bisphosphatase

30. What results in marasmus? What kind of muscle wasting? What kind of muscle wasting do you see in kwashiokor?
Hepatic fatty change
adrenal medulla
Trimethroprim inhibits bacterial dihyrofolate reductase
Loss of subcutaneous fat - tissue and muscle wasting (somatic muscle); visceral - not somatic

31. What has negative feedback inhibition on ribonucleoside reductase?
Chromosomal imbalance (downs for ex)
NADH - NADPH - FADH2
ATP and dATP
Alpha ketoglutarate; no TCA cycle intermediates

32. In the fasting state - What ensures that the pyruvate doesnt get used for TCA cycle?
2 oxidative and nonoxidative; cytoplasm (both); none (none produced either - you are simply switching carriers of energy- from glucose 6 phosphate to NADPH)
The products of FA oxidation (Acetyl CoA and NADH) phosphorylate pyruvate deyhdrogenase Which inactivates it
Double stranded RNA is created that is complementary to the mRNA of interest and When inserted into cells the strands separate and attach to the mRNA preventing it from being translated
Lack of lactase (brush border enzyme) so can not break down lactose (a dissaccharide of galactose and glucose); African Americans and Asians

33. What negatively regulates dopamine to norepinephrine?
Carbidopa
Water soluble vitamin stored in liver; converted to THF by dihydrofolate reductase and is a coenzyme in 1 methyl transfers - impt for DNA synth
2/3; LCAT (lecithin cholesterol acyltransferase)
Peripheral neuropathy - developmental delay - optic atrophy - globoid cells

34. What structures allows for degeneracy?
Vit C; Vit E
TRNA wobble
Smoking (FR takes up all the antioxidants); diets lacking fruit
Ascites from liver damage

35. What kind of linkages does glycogen synthase make? branching enzyme?
Lens and neural tissue; converts galactose to osmotically active galactitol
Block ATP synthase resulting in no ATP production and an increased proton gradient
Alpha 1 -4 linkages; alpha 1 -6 linkages
Abetalipoproteinemia and increased G3P production from all the carbs!

36. What is the importance of folic acid?
Citrate shuttle in the inner mitochondrial membrane transports acetyl coA into the cytoplasm; liver; liver and adipose tissue
Proprionyl coA; through methylmalonyl coA; B12
Def in Vit C; def in Vit D
Prevents neural tube defects in 1st trimester

37. What are the symptoms of arsenic poisoning?
Vomiting - rice water stools - garlic breath
Proprionyl coA; through methylmalonyl coA; B12
Chronic pancreatitis leads to type 1 diabetes - pancreatic insufficiency leads to ADEK deficiencies - malabsorption and steatorrhea - blocked gallbladder ducts leads to secondary biliary cirrhosis
Distinctive elfin faces - mental retardation - hypercalcemia d/t sensitivity to vit D - well developed verbal skills and extreme friendliness - CV problems

38. In What syndrome is HGPRT deficient? What is the inheritance pattern?
Lesch Nyhan syndrome; X linked recessive
Increased NADH causes lactate production and pyruvate dehydrogenase def causes increased lactate (d/t thiamine def that often occurs with alcoholics)
Phenylalanine build up
NADH - Acetyl CoA - ATP; NAD+ - ADP and Ca2+

39. What cofactor is required of preprocollagen hydroxylation?
Phenylalanine hydroxylase or tetrahydrobiopterin factor
Branched alpha ketoacid dehydrogenase
Glycogen and FFA oxidation - glucose conserved for final sprinting!
Vit C

40. What is the inheritance of Tuberous Sclerosis?
Branched chain alpha keto acid dehydrogenase; inability to break down branched chain aa
AD
Mcardles disease
Schilling test

41. What aa (other than glutamate) is a precursor for urea?
Wrinkles - acne and psoriasis topically - promyelocytic anemia - measles
is active
Carnitine shuttle
Arginine

42. What are the findings for Pompe's disease? What is the deficient enzyme?
Carbamoyl phosphate
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens
Inhibits lipoic acid; lipoic acid is a cofactor for pyruvate dehydrogenase
Cardiomegaly and systemic findings leading to early death; lysosomal alpha -1 -4- glucosidase (acid maltase)

43. What does actin/myosin participate in?
Phenylalanine hydroxylase; tetrahydrobiopterin factor
glucagon - insulin and epinephrine
Pompes
Microvilli - muscle contraction - cytokinesis - adherens junctions

44. What is one steroid that increases gluconeogenesis? What else does cortisol do that yields hyperglycemia?
They increase the permeability of the inner mit membrane resulting in a loss of gradient and loss of ATP production BUT electron transport continues! heat is generated instead
Lyonization; nondisjunction in mitotic divisions during embryonic period
Cortisol; inhibits insulin action
2 NADH - 2 ATP - 2 pyruvate - 2H+ - 2H20

45. What is the function of B2? What can cause deficiency?
Prolonged Ab use
Riboflavin makes up FAD and FMN both used in redox reactions; vegans
Citrate - Isocitrate - alpha ketoglutarate - Succinyl CoA - Succinate - Fumarate - Malate - Oxaloacetate
To increase serum Ca and phosphate for bone deposition

46. What are rotenon - CN - antimycin A - CO?
Pyruvate dehydrogenase; ATP - NADH - acetyl CoA
Wrinkles - acne and psoriasis topically - promyelocytic anemia - measles
Central and peripheral demyelination with ataxia and dementia
Block Electron transport

47. From What aa does dopa come from?
Branche aa alpha ketoacid dehydrogenase; thiamine; Isoleucine - leucine - valine
Tyrosine
Fomepizole; antidote for methanol or ethylene glycol poisoning
Pigmented iris hamartomas

48. Which anticancer drugs act on MT? how are they different?
Essential fructosuria and fructose intolerance; fructose intolerance
Cardiomegaly and systemic findings leading to early death; lysosomal alpha -1 -4- glucosidase (acid maltase)
Cortisol; inhibits insulin action
Vincristine - vinblastine - and paclitaxel; the vinca alkaloids block formation of the spindle and paclitaxel prevents breakdown of the mT spindle

49. What are two clinical symptoms of newborns with CF? and whats a lab value of a newborn with CF?
Alpha amanitin; mushroom caps
HVA
Meconium ileus and failure to thrive; negative serum immunoreactive trypsin
Debranching enzyme; alpha 1 - 4 glucosidase; no!; Pompes disease

50. How would one use a PCR to id an RNA virus?
Glycogen synthase
'9+2' 9 pairs of microtubules connected by an axonemal dynein ATPase in a cylinder with 20 MTs in the center; the connecting ATPase allows bending and differential sliding
Do an RT PCR (reverse transcriptase)- RT the RNA first - then PCR the DNA
SAM