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USMLE Step 1 Biochemistry

Subjects : health-sciences, usmle-step-1

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Answer 50 questions in 15 minutes.
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1. What explains the excess phenylketones in the urine in phenylketonuria?
Liver; berry aneurysms (d/t hypertension!) - and mitral valve prolapse
Phenylalanine build up
Down's Syndrome
Bendy bones seen in children due to decreased Vit D; craniotabes - skeletal deformities - growth retardation - rosary

2. What is the other name of Vitamin A? What is its source?
Retinol; Beta carotenes (dark and yellow vegetables)
Arg and his
Chorea and spastic paresis
Phosphorylation of glucose

3. What is the main form of glycogenolysis after glycogen phosphorylase? What happens instead in some lysosomes? does this process use glycogen phosphorylase? in What disease is this enzyme deficient?
Phospholipids - sphingolipids - glycolipids
Becomes methionine
Robertsonian translocations
Debranching enzyme; alpha 1 - 4 glucosidase; no!; Pompes disease

4. What lysosomal storage diseases are seen more in Ashkenazi Jew?
Dactinomycin - doxorubicin
Tay Sachs - Niemann Picks - Gauchers
Both add phosphate; phosphorylase uses no ATP and inorganic phosphate
Lactating mammary glands - adrenal cortex (sites of fatty acid or steroid synthesis) - RBCS (for glutathione reduction for FR injury)

5. Which vesicular trafficking protein takes vesicles from the Golgi to the ER (retrograde)?
Increased homocysteine in the urine - mental retardation - osteoporosis - marfanoid habitus - and atherosclerosis
COP I
AST (mit damage) - and GGT (d/t SER hyperplasia d/t induction of p450 generation)
Endonucleases - exonucleases - DNA polymerase - DNA ligase

6. why does galactose def present in newborns>
Lactose in mothers milk contains galactose
They use the H2O2 produced by some bacteria to produce bleach with myeloperoxidase Which they still have
NADPH- reductive biosynthesis!
Structural (constitutively active)

7. What is the rate determining enzyme in urea cycle?
Carnitine shuttle
SAM; Vit C
SER and mitochondria; NADPH
Carbomyl phosphate synthetase I

8. What is the source of ATP When you are fasting (in between meals)?
Hepatic glycogenolysis (mostly) - also hepatic gluconeogenesis - adipose release of FFA (minor)
Alkaptonuria; ochronosis
Galactose 1 phosphate uridyltransferase (GALT) results in an accumulation of toxic substances; failure to thrive - jaundice - hepatomegaly - infantile cataracts - mental retardation; exclude galactose and lactose from diet
Trimethroprim inhibits bacterial dihyrofolate reductase

9. Walk through the process that adipose tissue makes triacylglycerol.
Lipoprotein lipases take FA from VLDL and chylomicrons Which are then combined with glycerol 3 P (a carb intermediate) to make triacylglycerol
This depends on if phosphofructokinase 1 is active and this depends on whether or not fructose 2.6 bisphophate is activating it or not - this can only occur if PFK 2 is on and not FBPase 2. then fructose 6 phosphate will participate in glycolysis. Ot
As alanine; amino acids donate their amino group to alpha ketoglutarate making it glutamate and the aa then becomes an alpha ketoacid (joins TCA cycle); glutamate then converts pyruvate to alanine by donating its amino group to pyruvate (with ALT); a
Glutamate and aspartate; CO2 provides an ketone

10. What disease is associated with chromosome 16?
Catalase positive because they reduce their H2O2 and WBCs cant use that to make bleach; because the way they protect is by building a wall around the organisms with granulomas
By turning it into fructose - but sorbitol first; glucose to sorbitol via aldose reductase and sorbitol to fructose sorbital dehydrogenase
Type I collagen; AD
APCKD

11. What are the priorities in fasting and starvation?
To supply sufficient glucose to the brain and the RBCs and to preserve protein
Exons
When child gets two chromosomes from one parent; imprinting dx
Glucosekinase to glucose 6 P

12. What two substrates (other than pyruvate) does pyruvate carboxylase require?
Phosphoenolpyruvate; pyruvate kinase; fructose 1 - 6 BP; ATP - alanine
ATP and biotin
APCKD
Kidney; 1 alpha hydroxylase; Parathyroid hormone

13. What is the metabolism of fructose?
Cells that never got into G0 and divide rapidly with a short G1; bone marrow - gut epithelium - hair follicles - skin
Von gierkes
Fructose gets phosphorylated into fructose 1- P by fructokinase and then fructose 1P gets metabolized into DHAP or glyceraldehyde by aldolase B
Phosphofructokinase 1; fructose 1 phosphate to fructose 1 -6 bisphosphate; enhancers: AMP - fructose 2 -6 Bisphosphate; downregulators: citrate - ATP

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

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15. Insulin acts on __________ to deactivate glycogenolysis
Hemorrhage with increased PT and PTT time but normal bleeding time
both liver and muscle (unlike glucagon)
Disulfiram (antabuse); acetaldehyde accumulate contributing to hangover
Carbidopa

16. What type of enyzmes are used in base excision repair?
Myoclonus epilepsy
Chediak Higashi Syndrome; decreased phagocytosis results in recurrent pyogenic infections - partial albinism - and peripheral neuropathy
Glycosylases (only remove the base - cut the bond between ribose and base)
Cobalamin; cofactor for homocysteine methyltransferase (transfers SAM to THF to give to dUMP) and for methylmalonyl coA mutaseto make succinyl coA

17. What happens in RBCs after their membranes get oxidized?
Hemolytic anemia due to heinz bodies Which are oxidized hemoglobin precipitating within the RBCs
Do an RT PCR (reverse transcriptase)- RT the RNA first - then PCR the DNA
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
Orotate

18. What is a telangiectasia? Where can they be found?and What is a common result of them?
Neurologic defects - myopathy - lactic acidosis; giving ONLY ketogenic nutrient high fat content and aa (lysine and leucine)
Dilated capillaries and veins; skin - mucous membranes - GI - resp - urinary tracts; rupture and can cause epistaxis - hemorrhage - GI bleeding
An AP (apyrimidinic) endonuclease to cut the DNA - exonuclease then removes and then filled and sealed by enzymes
Arylsulfatase A; cerebroside sulfate

19. All glutamate goes to _______ d ammonia is used as a buffer in urine
When child gets two chromosomes from one parent; imprinting dx
kidney
Urine turns black on standing
homocysteine transferase

20. What three enzyme def. can cause homocysteinura? What is their mode of inheritance?
AR; trinucleotide deletion results in loss of a Phe aa on CFTR gene on chrom 7- the result is a mutated CFTR channel that gets degraded in the ER before reaching the surface; the purpose of CFTR is to increase Cl secretion in luminal secretions and r
Tryptophan; melatonin
Homocysteine methyltransferase - cystathionine reductase - decreased affinity of cystathionine reductase for pyridoxal phosphate; AR
Adrenal cells (steroid synthesis) - hepatocytes (p450 detox)

21. What are chromosomes? during What part of the cell cycle are they found? in What pathologic state would you see a lot of chromosomes?
Mcardles disease
Highly condensed forms of DNA; mitosis; neoplasia - highly dividing and thus hyperchromatic
4 -4 -9
To provide NADPH since there is an abundance of glucose 6 phosphate; also yields ribose for nucleotide synthesis and glycolysis intermediates

22. in Fabrys - What accumulates in tissue because fibroblasts express the def enzyme highly?
Ceramide trihexoside
Glucokinase
Alanine aminotransferase (ALT) and aspartate aminotransferase (AST); alanine to pyruvate and aspartate to oxaxloacetate
1) if it is stable/labile or permanent 2) intact BM and 3) intact ECM

23. What type exonuclease activity does DNA polymerase III have? in What organisms is it found?

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24. Where are cilia found? What symptoms are associated with kartageners syndrome?
In the reproductive tract and in the resp tract; infertility - bronchiectasis and recurrent sinusitis - associated with situs inversus
Proprionyl coA; through methylmalonyl coA; B12
To supply sufficient glucose to the brain and the RBCs and to preserve protein
Retinol; Beta carotenes (dark and yellow vegetables)

25. What enzyme is deficient in maple syrup urine disease?
Lime and CaCO3 to displace the niacin
Glycosylases (only remove the base - cut the bond between ribose and base)
Meiotic nondisjunction in mothers gametes during the first meiosis
Branched alpha ketoacid dehydrogenase

26. From What aa does serotonin come from? What NT is serotonin a precursor for?
Tryptophan; melatonin
Cortisol; inhibits insulin action
Steroid - cholesterol FA synthesis; HMP shunt
AD

27. What is the deficient enzyme in Fabry's disease? accumulated Substrate?
VMA
When the 2nd hit of the allele is What causes the disease; sporadic retinoblastoma
Alpha galactosidase A; ceramide trihexoside
Glutamine PRPP amidotransferase

28. other than succinate dehydrogenase What other enzymes are found in the inner mit membrane?
Glycerol goes to liver for gluconeogenesis; free fatty acids bind to albumin in blood and travel to cells to be used as for energy
ATP synthase - ATP- ADP translocase - ETC
Conversion to UDP glucose via UDP glucose pyrophosphorylase; glycogen synthase (rate limiting step )
Via fructose 1 P by fructokinase in the liver; as glucose 1 by being GALT- ed ;)

29. What is acyl coA dehydrogenase used in? What does a deficiency in this enzyme produce?
Dynein - retrograde - kinesin - anterograde
Meconium ileus and failure to thrive; negative serum immunoreactive trypsin
A 16C FA! the product in FA synthesis; saturated; no double bonds
Fatty acid (acyl coA) oxidation to acetyl coA; increase in dicarboxylic acids - and decreased glucose and ketones

30. How is CF diagnosed?
Chorea and spastic paresis
Gets converted to pyruvate via the Cori cycle in the liver
Malnutrition - edema (hypoalbuminemia) - anemia liver (fatty change) - diarrhea d/t no brush border enzymes (no protein!)
Increased concentration of Cl ions in sweat test

31. Other than anabolic processes What else is NADPH used for?
Respiratory burst - P450 - and glutathione reductase
Inbit Na+K+ ATPase; Cardiac glycosides
Uracil; thymine
Fruits and vegetables

32. Other than ragged red fibers and lactic acidosis - What else is common in mit myopathies?
Neurologic lesions and cardiomyopathies (ox phos!)
Increased CPK and muscle biopsy
AD (trinucleotide expansion is not a mode of inheritance)
can be d/t lyonization

33. What antibiotic inhibits peptidyltransferase activity?
Pantothenate; essential comp of CoA (needed for acyl transfers) and fatty acid synthase
Chloramphenicol
Succinyl coA
Niemann pick and Tay Sachs; hepatosplenomegaly or not

34. Which cell types have a high level of RER?
Essential fructosuria and fructose intolerance; fructose intolerance
Downs
Mucus secreting goblet cells - ab secreting plasma cells - nt secreting neurons
HnRNA; mRNA

35. From What aa does glutathione come from?
Branched alpha ketoacid dehydrogenase
FISH
Phosphofructokinase 1; fructose 1 phosphate to fructose 1 -6 bisphosphate; enhancers: AMP - fructose 2 -6 Bisphosphate; downregulators: citrate - ATP
Glutamate

36. What aa is the precursor for NO?
An aminotransferase enzyme transfers an amino group from an amino acid on to alpha ketoglutarate making it a glutamate (to join the urea cycle or excreted in the kidney) and the amino acid that lost the amino group is then turned into a glycolytic in
glycogenolysis
2 NADH - 2 ATP - 2 pyruvate - 2H+ - 2H20
Arginine

37. What does adipose tissue contribute for hepatic gluconeogenesis during starvation? muscle?
AAUAAA; Poly A polymerase; no
Glycerol and propionyl coA; lactate and alanine
NADPH oxidase; CGD
Fatty acid (acyl coA) oxidation to acetyl coA; increase in dicarboxylic acids - and decreased glucose and ketones

38. What are two reasons why pyruvate would be pushed to lactate in an alcoholic?
Chloramphenicol
Shiga toxin and ricin (protein in castor beans)
Flourescent DNA/RNA In SITU hybridization; for visualization of anomalies (like microdeletions) that are too small to be seen on karyotype; Prader - Willi syndrome
Increased NADH causes lactate production and pyruvate dehydrogenase def causes increased lactate (d/t thiamine def that often occurs with alcoholics)

39. What are the symptoms of B2 deficiency?
Cheilosis and corneal vascularization
AR
Carbamoyl phosphate to orotic acid
Neural tissue and adrenal medulla

40. What converts dUMP to dTMP? What drug blocks this?
Thymidylate synthase; 5- Flourouracil
Myotonic Dystrophy (CTG) - Huntington (CAG) Friedricks Ataxia (GAA) - Fragile (CGG)
Iatrogenic - granulomatous disease (sarcoidosis d/t epitheloid macrophage release of calcitriol); hypercalcemia - hypercalciuria (renal calculi!) - loss of appetite - stupor
No protein!

41. _______ is ubiquitous
It inhibits conversion of DHF to THF by blocking dihyrdofolate reductase
hexokinase (glucokinase only found in liver and beta cells of pancreas)
5'--> 3'; prokaryote only; for removing RNA primer and filling it with DNA
in hepatocytes

42. How is Vit K activated? What drug blocks this? What is the source of Vit K?
Dynein - retrograde - kinesin - anterograde
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables
Cobalamin; cofactor for homocysteine methyltransferase (transfers SAM to THF to give to dUMP) and for methylmalonyl coA mutaseto make succinyl coA
Phenylketones that build up in phenylketonuria

43. What causes the pathology in I- cell disease? What does I cell sidease
Decreased serum phosphate
Via fructose 1 P by fructokinase in the liver; as glucose 1 by being GALT- ed ;)
The lack of lysosomal proteins (are secreted - default from RER!) results in macromolecules accumulating in lysosomes - inclusions cause pathology
can be d/t lyonization

44. What enzyme is involved in the degradation of tyrosine to fumarate?
golgi apparatus
Vimentin - desmin - cytokeratin - glial fibrillary acidic protein (GFAP) - neurofilaments; to differentiate between different cells
Homogentistic acid oxidase
Folinic acid; does not require DHF

45. How do hepatocytes release glucose 6 P from glycogenolysis into the blood ? in What disease is this enzyme deficient?
With glucose 6 phosphatase; Von Gierkes disease
Macrocytic megaloblastic anemia; it is the most common vit def in the US; no neuro symptoms and no methylmalonyl coA in the urine
aldose reductase
No; increased mutation rate!

46. What does a deficiency cause?
Each codon specifies only one amino acid
Night blindness - dry skin
Arylsulfatase A; cerebroside sulfate
Metaphase (all lined up); blood - bone marrow - amniotic fluid - placental tissue; used to diagnose chromosomal abnormalities (deletions - trisomies - sex chromosomal disorders)

47. AR disease are usually...
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)
enzymatic
DHF
Genotyping for mutant alleles (different sizes)

48. What is the difference between a kinase and a phophorylase?
Both add phosphate; phosphorylase uses no ATP and inorganic phosphate
Also have fair skin - blond hair - blue eyes - eczema
teratogenic
Dopa decarboxylase; B6

49. After N5N10 methylene THF donates a methyl for conversion of dUMP to dTMP What is it converted to?
Homogentistic acid oxidase
calorie deficient
Niacin/Nicotinic acid; NAD+ and NADH used in redox reactions - and catabolic and anabolic reactions; B6; tryptophan
DHF

50. What does vit C excess cause?
Gauchers; hepatosplenomegaly - aseptic necrosis of the femur - bone crises - Gauchers cells - crumpled tissue paper looking (fibrillar appearing macrophages in liver - spleen - b.m.)
AR; trinucleotide deletion results in loss of a Phe aa on CFTR gene on chrom 7- the result is a mutated CFTR channel that gets degraded in the ER before reaching the surface; the purpose of CFTR is to increase Cl secretion in luminal secretions and r
Renal calculi of uric acid (replaces uric acid in excretion)
In the reproductive tract and in the resp tract; infertility - bronchiectasis and recurrent sinusitis - associated with situs inversus

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USMLE Step 1 Biochemistry

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