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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.
Don't refresh. All questions and answers are randomly picked and ordered every time you load a test.

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 dysgeusia?
Either salvage by HGPRT and PRPP or converstion to xanthine and then to uric acid by xanthine oxidase
Triacylglycerols in adipose tissue broken down into glycerol and free fatty acids; growth hormone and epinephrine
Inability to taste
Pataus

2. Give examples of the anabolic processes that NADPH participates in. From What process is NADPH produced?
Later
Do agarose gel eletrophoresis to separate the different sizes of the PCR products (sizes id the the fragment)
Hereditary defect of renal tubular amino acid transporter for cysteine - ornithine - lysine and arginine in the PCT of kidnyes
Steroid - cholesterol FA synthesis; HMP shunt

3. What is the result of arginase deficiency?
Glycogen synthase
Chorea and spastic paresis
teratogenic
Glutamine PRPP amidotransferase

4. What is the mode of inheritance for the multiple endocrine neoplasias?
Cardiomegaly and systemic findings leading to early death; lysosomal alpha -1 -4- glucosidase (acid maltase)
AD
Neural tissue and adrenal medulla
Prevents neural tube defects in 1st trimester

5. Do RNA polymerases have proofreading function? What does this mean for HIV?
No; increased mutation rate!
Citrulline is combined with aspartate to make arginosuccinate Which is then split into fumarate (which leaves to enter TCA) and arginine. Arginine is then converted to Ornithine (via arginase) giving off Urea Which then goes to the kidney and ornithi
Inbit Na+K+ ATPase; Cardiac glycosides
Cafe au lait spots - optic pathway gliomas - lisch nodules - scoliosis - neural tumors

6. Which aa are only ketogenic?
Lactose in mothers milk contains galactose
Lysine and leucine
Southern - DNA - Northern - RNA - Western - Protein; blotting procedures are used to detect a specific nucleic acid or aa sequence; the sample is electrophoresed and then transferred to a filter Which is then soaked in a denaturant and then a radiola
GTP: gluconeogenesis - OXA to phosphoenolpyruvate

7. Why do mitochondrial diseases exhibit variable expression?
Heteroplasmy
Fat malabsorption (CF) and abetalipoproteinemia (damage takes up all antioxidants)
Dopamine beta hydroxylase; vit C
Acetyl coA and NADH inhibit pyruvate dehyrogenase

8. What is the deficient enzyme in Fabry's disease? accumulated Substrate?
Alpha galactosidase A; ceramide trihexoside
SAM; Vit C
Very Poor Carb Metabolism (Von Gierke - Type 1 - Pompes - Type II - Cori's - Type III - McArdles disease - Type IV)
Increased homocysteine in the urine - mental retardation - osteoporosis - marfanoid habitus - and atherosclerosis

9. What is the difference between rickets and osteomalacia?
1) Dry beriberi: peripheral neuropathy demyelination (symmetricul muscle wasting) 2) Wernicke: ataxia - confusion - nystagmus - mammilary body and medial dorsal nuc of thalamus hemorrhage 3) Korsakoff: anterograde and retrograde amnesia - demyelinati
Increased concentration of Cl ions in sweat test
Osteomalacia is vit D def seen in adults and causes bowed legs and results in increased fractures
A folic acid analogue

10. No offspring of affected males with a mit disease can...
Phenylalanine hydroxylase; tetrahydrobiopterin factor
show disease
Flouroquinolones
Heart - SKELETON - and EYES: arachnodactyly - long limbs - cystic medial necrosis of aorta resulting in aneurysms (and eventual dissection) - floppy mitral valve (prolapse and regurg!) - subluxation of lens

11. What are the general symptoms of 22q11 chromosomal deletion? why is it that these occur?
Hemolytic anemia with a blood smear with heinz bodies and bite cells (phagocytic removal of heinz bodies)
Macrocytic megaloblastic anemia - hypersegmented PMNs - subacute combined degeneration myelopathy Which results in degeneration of the dorsal and lateral columns (loss of position and vibration sense - spastic paresis and ataxia) due to abnormal myel
Galactose gets converted to galactose 1 Phosphate by galactokinase Which then gets turned into glucose 1 phosphate by Galactose 1 Uridyl Transferase (GALT) Which also produces UDP Gal Which is used for lactose production in the breast; the fate of gl
Cleft palate - Abnormal faces - thymic aplasia (t cell def) - Cardiac defects - hypocalcemia (secondary to hypoparathyroid); 3rd and 4th branchial pouch aberrant development

12. Name two diseases that have to do with elastin; discuss their pathogenesis.
Marfans and emphysema; marfans is a fibrillin protein defect - fibrillin is a supporting protein in tropoelastin scaffolding and results in aorta - skeletal and eye defects; emphysema is due to a deficiency in the function of alpha 1 antitrypsin
HMG CoA synthetase; HMG CoA reductase
Uracil; thymine
Lysine and arginine; basic

13. What two compounds provide the two N's of urea? What else is in urea other than two NH2's?
Ouabain
Branching increases the rate of synthesis and breakdown of glycogen by having more sites to add on to and break off
Reserve supply of glucose in the fasting state - liver gives to the blood and skeletal muscle uses for itself
Glutamate and aspartate; CO2 provides an ketone

14. What is the inheritance pattern of hereditary spherocytosis?
B hydroxybutyrate - acetone - acetoacetate; beta hydroxybutyrate; beta hydroxybutyrate
UV specific endonucleases for excision repair; AR; dry skin - photosensitivity - poikiloderma - susceptibility to skin cancers
AD
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)

15. Which cell types have a high level of RER?
Mucus secreting goblet cells - ab secreting plasma cells - nt secreting neurons
Zero order; increased production of NADH no matter the amount of end products produced
Fatty acid (acyl coA) oxidation to acetyl coA; increase in dicarboxylic acids - and decreased glucose and ketones
Urea cycle enzyme deficiency (mc: ornithine transcarbamoylase); liver disease - alcoholic cirrhosis and reye's

16. What is def of Vit E caused by?
Cardiomegaly and systemic findings leading to early death; lysosomal alpha -1 -4- glucosidase (acid maltase)
Robertsonian translocations
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
Fat malabsorption (CF) and abetalipoproteinemia (damage takes up all antioxidants)

17. How would one use a PCR to id an RNA virus?
Thymidylate synthase; 5- Flourouracil
Do an RT PCR (reverse transcriptase)- RT the RNA first - then PCR the DNA
Decreased phenylalanine - increase tyrosine
Tyrosine

18. From What aa does dopa come from?
Consumption of bear liver - and overtreatment isoretinoin; constitutional symptoms and alopecia and skin changes
jaundice
Tyrosine
Def in Vit C; def in Vit D

19. What are two examples of X linked Autosomal Dominant disease?
Enriched whole grain cereals and other things; thiamine; malnutrition (nonenriched white processed bread and rice) and alcohol second to malnutrition
Hypophosphatemia rickets and Alports
Positive on phosphofructokinase 1 and negative on fructose 1 -6 bisphosphatase
Glycogen synthase

20. Is DNA acidic or basic?
Meiotic nondisjunction in mothers gametes during the first meiosis
Acidic
Increased nuchal translucency
Matrix; intermembrane space

21. What toxin causes liver failure by inhibiting RNA polymerase II if ingested? Where does it come from?
Alpha amanitin; mushroom caps
Some viruses
can be d/t lyonization
Homocysteine methyltransferase - cystathionine reductase - decreased affinity of cystathionine reductase for pyridoxal phosphate; AR

22. Which form of iron is most absorbable? What form is non heme ingested iron in (vegetables for ex)?
Increased G3P for TG synthesis - increased acetyl CoA (Fatty acid synthesis for TG synthesis) - and decreased oxidation of FA in the mitochondria due to alcohols toxicity on the mit
Fe2+; Fe3+
Lactic acidosis and ketoacidosis
NADH - Acetyl CoA - ATP; NAD+ - ADP and Ca2+

23. What happens to S adenosyl methionine after it donates a methyl? What enzyme regenerates methionine? What two factors are necessary to do this? who gives the methyl?
In hepatocytes; B6
S adenosyl homocysteine; homocysteine transferase; B12 and THF; M- THF
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
Peripheral neuropathies of hands/feet; angiokeratomas between umbilicus and knees; cardiovascular/renal disease

24. What is the other name for Vitamin B5? What is its function?
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis
Lysine and arginine; basic
Reserve supply of glucose in the fasting state - liver gives to the blood and skeletal muscle uses for itself
Pantothenate; essential comp of CoA (needed for acyl transfers) and fatty acid synthase

25. What is the importance of folic acid?
Uses HMG coA reductase
COP II
Prevents neural tube defects in 1st trimester
Via fructose 1 P by fructokinase in the liver; as glucose 1 by being GALT- ed ;)

26. What is the precursor for pyrimidines?
Orotate
Cells that never got into G0 and divide rapidly with a short G1; bone marrow - gut epithelium - hair follicles - skin
hexokinase - glucokinase (its a glutton)
Liver; P450 hydroxylation

27. Where is type 1 collagen found?
Bone - Skin - Tendon - dentin - fascia - cornea - late wound repair
By cyclin CDK complexes
tyrosine hydroxylase
To transfer methyls; a methionine and an ATP; norepinephrine plus a methyl makes epinephrine

28. What else decreases production of oxaloacetate in alcoholism other than decreased pyruvate?
In hepatocytes; B6
Oxaloacetate gets shifted to malate d/t excess NADH
Decreased phenylalanine - increase tyrosine
Carbamoyl phosphate to orotic acid

29. Glucokinase is a...
Mitosis then Interphase (G1 - S - G2); in G1 is presynthesis phase - S is synthesis phase (DNA rep. etc) - G2 is premitotic (making of MT spindle etc)
fatty acid degradation
glutton
Meconium ileus and failure to thrive; negative serum immunoreactive trypsin

30. What causes hypoglycemia in alcoholism?
Lime and CaCO3 to displace the niacin
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens
Hereditary defect of renal tubular amino acid transporter for cysteine - ornithine - lysine and arginine in the PCT of kidnyes
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis

31. What are causes of zinc deficiencies?
Alcoholism - RA - inflammatory disease - chronic diarrhea
H1
Induction in chronic alcoholism (increases tolerance); inhibition in acute alcoholism
adrenal medulla

32. What is the rate limiting step in fatty acid synthesis? What four things inhibit this rate limiting step? What stimulates it?
Vomiting - rice water stools - garlic breath
Lens and neural tissue; converts galactose to osmotically active galactitol
Acetyl coA carboxylase converting Acetyl coA into malonyl coA; increased AMP - glucagon - epinephrine - palmitate; citrate
Make it stiffer/more tensile; continues as we get older so gets less and less elastic

33. What type of enyzmes are used in base excision repair?
Indirect inhibtion of the Na+/Ca+2 exchange resulting in increased intracellular calcium Which increases cardiac contractility
Glycosylases (only remove the base - cut the bond between ribose and base)
Increased concentration of Cl ions in sweat test
No; increased mutation rate!

34. ____ soluble vitamins are more likely to result in toxicity
fat
Glutamate
Schwann cells - lens - retina - kidneys because they only have aldose reductase and no sorbital dehydrogenase
Dopamine beta hydroxylase; vit C

35. What is the presentation of G6PD deficiency?
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!!
Citrate - Isocitrate - alpha ketoglutarate - Succinyl CoA - Succinate - Fumarate - Malate - Oxaloacetate
No
Hemolytic anemia with a blood smear with heinz bodies and bite cells (phagocytic removal of heinz bodies)

36. What is the treatment for cystinuria?
Tetrahydrofolates
Inosine/IMP; adenosine deaminase
Acetazolamide to alkalinize the urine
Dark connective tissue - brown pigmented sclera - debilitating athralgias; homogentistic acid builds up and can be toxic to cartilage

37. What is hartnup disease? What is the mode of inheritance?
No; increased mutation rate!
Defective neutral amino acid transporter on renal and intestinal cells; AR
Using serine (which turns into glycine after reaction)
Progressive neurodegeneration (spasticity d/t UMN disease) - hepatosplenomegaly - cherry red spot on macula - foam cells

38. Where is anaerobic glycolysis used often?
Because that is Where the products will be consumed; acetyl coA; TCA cycle of ketone body production
False; GTP hydrolysis
Beckers is less severe (instead of deletion is either defect or deficiency) with later presentation
RBCs - renal medulla - leukocytes - lens - testes - cornea

39. What does guanine have on it?
Nitric Oxide - creatinine - Urea
A ketone
Telomerase; an RNA polymerase that uses a RNA template to add DNA to the end of shortening 3' ends
Which dimerizes and activates a protein phosphatase Which de phosphorylates glycogen phosphorylase

40. What are the clinical findings in edwards syndrome?
APCKD
Renal calculi of uric acid (replaces uric acid in excretion)
To supply sufficient glucose to the brain and the RBCs and to preserve protein
Mental retardation - rocker bottom feet - micrognathia (small jaw) - low set ears - clenched hands with polydactyly - congenital heart disease

41. What is alternative splicing? in What disease state does alt splicing occur unintentionally d/t early stop codons?
Alcohol dehyrogenase (alcohol to acetaldehyde); acetaldehyde dehydrogenase (acetaldehyde to acetate); NAD+; B3 (niacin)
Galactokinase; galactose appears in blood and urine - sometimes infantile cataracts may present as failure to track objects or lack of a social smile
When different combination of exons get combined together; in Beta thallasemia
Bacterial synthesis in the intestine

42. From What aa does niacin come from? with What vitamin cofactor?
Each codon specifies only one amino acid
Increased concentration of Cl ions in sweat test
usually present late
Tryptophan; B6

43. What is PRPP? How is it made?
Vincristine - vinblastine - and paclitaxel; the vinca alkaloids block formation of the spindle and paclitaxel prevents breakdown of the mT spindle
Respiratory burst - P450 - and glutathione reductase
Phosphofructokinase 1
Its the ribose; its made from PRPP synthetase using ribose 5 P from the HMP shunt

44. The result of dephosphorylation of PFK2 is...
activation of PFK1
Alanine aminotransferase (ALT) and aspartate aminotransferase (AST); alanine to pyruvate and aspartate to oxaxloacetate
Either salvage by HGPRT and PRPP or converstion to xanthine and then to uric acid by xanthine oxidase
Niemann pick and Tay Sachs; hepatosplenomegaly or not

45. What are the symptoms of B2 deficiency?
Loss of subcutaneous fat - tissue and muscle wasting (somatic muscle); visceral - not somatic
Cheilosis and corneal vascularization
No protein!
Arylsulfatase A; cerebroside sulfate

46. What does actin/myosin participate in?
Lactose (glucose and galactose)
Microvilli - muscle contraction - cytokinesis - adherens junctions
Gets converted to pyruvate via the Cori cycle in the liver
Macrocytic megaloblastic anemia; it is the most common vit def in the US; no neuro symptoms and no methylmalonyl coA in the urine

47. What disease is associated with chromosome 13?
Pataus
Tyrosinase deficiency; defective tyrosine transporters; lack of migration of neural crest cells; skin cancer
Vit C
To increase serum Ca and Phosphate for ultimate bone deposition by: increase intestinal absorption of calcium and phosphate (also renal); and increase resorption of bone by stimulating macrophages (osteoclasts)

48. What is the source of ATP after a meal?
An AP (apyrimidinic) endonuclease to cut the DNA - exonuclease then removes and then filled and sealed by enzymes
An allosteric regulator; fructose 1 phosphate; phosphofructokinase 2
Double strand breaks; nonhomologous end joining
Glycolysis and aerobic respiration

49. Other than ragged red fibers and lactic acidosis - What else is common in mit myopathies?
Zero order; increased production of NADH no matter the amount of end products produced
usually present late
Acetazolamide to alkalinize the urine
Neurologic lesions and cardiomyopathies (ox phos!)

50. What is the inheritance pattern of hereditary hemorrhagic telangiectasia or Olser Weber Rendu Syndrome? What is it?
FISH
AD; disorder of blood vessels that results in telangietasias - skin discolorations - and AVMs (arteriovenous malformations)
only L form
Meconium ileus and failure to thrive; negative serum immunoreactive trypsin