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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. The ________ is 50% cholesterol - 50% phoshpolipids
Dopamine beta hydroxylase; vit C
Its When two acrocentric chromosomes combine their long arms (and short arms disappear); When the translocation is functional
Pellagra
plasma membrane

2. IMP is degraded to inosine and then to hypoxanthine. What two options are next? by What enzymes?
Fat malabsorption (CF) and abetalipoproteinemia (damage takes up all antioxidants)
Homocysteine methyltransferase - cystathionine reductase - decreased affinity of cystathionine reductase for pyridoxal phosphate; AR
1 for all three classes of RNA; Rifampin; TB
Either salvage by HGPRT and PRPP or conversion to xanthine and then uric acid by xanthine oxidase

3. What is the exception to the universal aspect of the genetic code?
6MP blocks de novo purine synthesis
Mitochondria - archaebacteria - mycoplasma - and some yeasts
X linked recessive; deletion of the dystrophin gene (connects actin to the membrane glycoprotein) due to frame shift mutation - lack of dystrophin results in muscle breakdown Which gets replaced by fibrofatty tissue and collagen
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis

4. What is the function of mitDNA?
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
Encode enzymes for oxphos mit reactions
Later
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis

5. What is the purpose of HMP shunt?
Down's Syndrome
Respiratory burst - P450 - and glutathione reductase
Acetone
To provide NADPH since there is an abundance of glucose 6 phosphate; also yields ribose for nucleotide synthesis and glycolysis intermediates

6. What disease is associated with chromosome 5?
can be d/t lyonization
is active
Cri du Chat
ATP and biotin

7. Prokaryotes have What ribosomal units?
Myotonic Dystrophy (CTG) - Huntington (CAG) Friedricks Ataxia (GAA) - Fragile (CGG)
30S + 50S= 70S
Either salvage by HGPRT and PRPP or conversion to xanthine and then uric acid by xanthine oxidase
TPP (from B1- pyrophosphate) - FAD (B2) - NADH2 (B3) - CoA (B5 - panothenate) - Lipoic Acid

8. What else becomes cystic often in APCKD? other than cystic presentations What else do APCKD patients present with?
Vincrstine (vinca alkaloids) and paclitaxel
Succinate hydrogenase; matrix
Hemolytic anemia (fragility of RBCs) - muscle weakness and posterior column and spinocerebellar tract demyelination (neuron damage) resulting in decreased proprioception and vibration sensation - and peripheral neuropathy - and retinal degeneration
Liver; berry aneurysms (d/t hypertension!) - and mitral valve prolapse

9. The main hormonal regulation of glycogen is...
Tay Sachs - Niemann Picks - Gauchers
Arg and his
glucagon - insulin and epinephrine
Myoclonus epilepsy

10. What type of exonuclease activity does DNA polymerase I have? in What organisms is it found? What is it used for?

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11. What is retinoic acid used for in treatments?
Glutamine PRPP amidotransferase
NADH - Acetyl CoA - ATP; NAD+ - ADP and Ca2+
Gain of function gene after expansion results in overproduction of huntingtin protein that in overproduction results in toxicity to neurons in the caudate --> caudate atrophy and decreased levels of GABA and Ach
Wrinkles - acne and psoriasis topically - promyelocytic anemia - measles

12. The __________ adds O linked oligosaccharides to serine and threonine residues on proteins
Phenylalanine hydroxylase; tetrahydrobiopterin factor
Dopa decarboxylase; B6
golgi apparatus
Succinate hydrogenase; matrix

13. What is the rate determining enzyme in glycogenolysis?
Yes! congenital simply means 'born with'- hereditary is familial
Glycogen phosphorylase
IMP
Because cells of mononuclear phagocytic are esp rich in lysosomes

14. What carries electrons (as energy)?
Mannose
Reticulin - skin - blood vessels - uterus - fetal tissue - early phase of granulation tissue
Tyrosinase
NADH - NADPH - FADH2

15. What does the TCA cycle produce?
A folic acid analogue
Genotyping for mutant alleles (different sizes)
3 NADH - 1 FADH2 - 2 CO2 - 1 GTP = 12 ATP/acetyl CoA
RER in nucleus (synthesize NTs)

16. What is the link between adenosine deaminase deficiency and decreased lymphocyte count?
S adenosyl homocysteine; homocysteine transferase; B12 and THF; M- THF
Excess ATP and dATP has negative feedback inhibition on ribonucleotide reductase Which results in decreased DNA synthesis and thus decreased lymphocyte count
Carbomyl phosphate synthetase I turns N acetyl glutamate into carbomyl phosphate and then ornithine transcarbomyalse turns carbomyl phosphate into citrulline (using ornithine)
cell cycle events

17. What is ascorbic acid? What is its 5 main function?
Each codon specifies only one amino acid
Via fructose 1 P by fructokinase in the liver; as glucose 1 by being GALT- ed ;)
Osmotic diarrhea - bloating - cramps
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. AD disease...
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs
XR - iduronate sulfatase
usually present late
Either salvage by HGPRT and PRPP or converstion to xanthine and then to uric acid by xanthine oxidase

19. What structures allows for degeneracy?
In ability to reabsorb phosphate from renal tubular cells results in osteomalacia due to defective bone mineralization; no
Carbomyl phosphate synthetase I
TRNA wobble
In hepatocytes; B6

20. What is osmotic damage and What molecules cause it? give some examples of osmotic damage - in What disease state do you see these?
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
Either salvage by HGPRT and PRPP or converstion to xanthine and then to uric acid by xanthine oxidase
Renal failure - decreased sun - fat malabsorption - chronic liver disease - p450 induction - hypoparathyroidism (activates 1 alpha hyrdoxylase)
When water is pulled in; polyols (sugar alcohols); cataracts - retinopathy - peripheral neuropathy all seen in chronic hyperglycemia in diabetes

21. Which amino acids are basic? are they negatively or positively charged at body pH?
AD: yes! most common presentation; very rare (700 mg/dl cholesterol level!)
Arg - lys - his; positive except His has no charge at body pH
Carbamoyl phosphate to orotic acid
Thymidylate synthase; 5- Flourouracil

22. What is Pleiotropy? What is the opposite of Pleiotropy? Give an example of each.
ATP is an allosteric inhibitor on both phosphofructokinase and pyruvate kinase and pyruvate hydrogenase; and fructose 2 -6 BP Which is an allosteric enhancer on phosphofructokinase is an allosteric downregulator on fructose 1 -6 bisphophatase
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)
Homocysteine methyltransferase - cystathionine reductase - decreased affinity of cystathionine reductase for pyridoxal phosphate; AR
Both AD

23. why does a deficiency in ornithine transcarbamoylase (urea cycle enzyme) result in an increased production of orotic acid - the pyrimidine precursor?
Fructose 1 -6 bisphosphatase
Cardiomegaly - cyanosis - tachycardia - dyspnea - vomitting; 2-3 months
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis
AD: yes! most common presentation; very rare (700 mg/dl cholesterol level!)

24. What are two examples of X linked Autosomal Dominant disease?
Mental retardation - obesity - short stature - hypogonadism - hypotonia - hyperphagia; father
Macroorchitism - big ears - big jaw - mental retardation (2nd mcc!) - mitral valve prolapse - long face
Vitamin B1 (thiamine) - Vitamin B2 (riboflacin) - Vitamin B3 (niacin) - Vitamin B5 (pantothenate) - Vitamin B6 (pyridoxine) - Vitamin B7 (Biotin) - Vitamin B12 (cobalamin) - folate - Vit. C
Hypophosphatemia rickets and Alports

25. What causes an excess of B3? What are the symptoms?
Niacin treatment for hyperlipidemia; facial flushing
G6PD; provides malarial resistance; X linked recessive
IMP
Cleft palate - Abnormal faces - thymic aplasia (t cell def) - Cardiac defects - hypocalcemia (secondary to hypoparathyroid); 3rd and 4th branchial pouch aberrant development

26. What Ab class blocks DNA gyrase?
Flouroquinolones
Mcardles disease
32 or 30; malate aspartate shuttle or glycerol 3 phosphate shuttle; heart/liver and muscle
Debranching enzyme; alpha 1 - 4 glucosidase; no!; Pompes disease

27. What is the reason for blue sclerae in Osteogenesis Imperfecta?
Adrenal cells (steroid synthesis) - hepatocytes (p450 detox)
Thinned cornea from lack of connective tissue so see underlying choroidal lens
Neurologic defects - myopathy - lactic acidosis; giving ONLY ketogenic nutrient high fat content and aa (lysine and leucine)
Glutamate and aspartate; CO2 provides an ketone

28. Which vitamin keeps Fh4 in its reduced form?
Biotin; rate limiting step in fA synthesis - acetyl coA into malonyl coA
Vit C
Oxaloacetate gets shifted to malate d/t excess NADH
Citrate shuttle in the inner mitochondrial membrane transports acetyl coA into the cytoplasm; liver; liver and adipose tissue

29. What is the rate determining enzyme in steroid synthesis? cholesterol synthesis?
Chorea and spastic paresis
Hemolytic anemia; jaundice in newborn if mom gets too much vit K
HMG CoA synthetase; HMG CoA reductase
Uses HMG coA reductase

30. Where does a majority of galactose in our diet come from?
Carbomyl phosphate synthetase I turns N acetyl glutamate into carbomyl phosphate and then ornithine transcarbomyalse turns carbomyl phosphate into citrulline (using ornithine)
Lactose (glucose and galactose)
Alpha amanitin; mushroom caps
When child gets two chromosomes from one parent; imprinting dx

31. What is the rate determining enzyme in fructose metabolism?
Asp and glu; negatively
Complex I - Complex II (lower energy)
Lack of lactase (brush border enzyme) so can not break down lactose (a dissaccharide of galactose and glucose); African Americans and Asians
Aldolase B

32. What is required to get Acyl coA in FA degradation into the mitochondria for beta oxidation to acetyl coA groups?
Carnitine shuttle
A methyl
Prolonged Ab use
2 -4 dinitrophenol - aspirin - and thermogenin (in brown fat)

33. What does FISH stand for? What is it used for? Give an example.
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -
Flourescent DNA/RNA In SITU hybridization; for visualization of anomalies (like microdeletions) that are too small to be seen on karyotype; Prader - Willi syndrome
Biotin; rate limiting step in fA synthesis - acetyl coA into malonyl coA
AD

34. What is the main aa composition of collagen? What other 2 aa are common?
Branched alpha ketoacid dehydrogenase
Glycine; proline and lysine
To transfer methyls; a methionine and an ATP; norepinephrine plus a methyl makes epinephrine
Increased concentration of Cl ions in sweat test

35. 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?
Chronic pancreatitis leads to type 1 diabetes - pancreatic insufficiency leads to ADEK deficiencies - malabsorption and steatorrhea - blocked gallbladder ducts leads to secondary biliary cirrhosis
S adenosyl homocysteine; homocysteine transferase; B12 and THF; M- THF
Pantothenate; essential comp of CoA (needed for acyl transfers) and fatty acid synthase
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

36. What is a common result of unbalanced robertsonian translocation?
Progressive neurodegeneration (spasticity d/t UMN disease) - hepatosplenomegaly - cherry red spot on macula - foam cells
No; increased mutation rate!
Thinned cornea from lack of connective tissue so see underlying choroidal lens
Chromosomal imbalance (downs for ex)

37. What disease is associated with chromosome 18?
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
Exons
H1
Edwards

38. What is the treatment for phenylketonuria?
hexokinase (glucokinase only found in liver and beta cells of pancreas)
Decreased phenylalanine - increase tyrosine
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
Citrate - Isocitrate - alpha ketoglutarate - Succinyl CoA - Succinate - Fumarate - Malate - Oxaloacetate

39. What does methotrexate do? What step is this enzyme involved in?
Codons may encode same amino acid since there are only ~20 aa
Mental and growth retardation; seizures - fair skin - eczema - musty body odor; neurotoxic ketones and acids stimulate vomitting - can lead to pyloric stenosis
It inhibits conversion of DHF to THF by blocking dihyrdofolate reductase
Increased orotic acid in urine - megaloblastic anemia - +/- hyperammonemia

40. After addition of an aa - Where does the aminoacyl tRNA now bind to?
DHF
30S ribosomal RNA (small one)
Galactokinase deficiency and classic galactosemia; classic galactosemia
Dopa decarboxylase; B6

41. Is heterochromatic more accessible or less accessible to TF? euchromatin?
MT!
Malnutrition - edema (hypoalbuminemia) - anemia liver (fatty change) - diarrhea d/t no brush border enzymes (no protein!)
Branching increases the rate of synthesis and breakdown of glycogen by having more sites to add on to and break off
Less; more

42. What is the rate determining enzyme in HMP shunt?
Glucose 6 phosphate dehydrogenase
Ornithine transcarbomyalse; orotic aciduria; X linked recessive; autosomal recessive
VHL (Von Hippa Lindau)
They can enter G1 from G0 and replicate if they like; hepatocytes - lymphocytes - fibroblasts

43. Which oxidant is most attributable to damage of RBCs?
Acetyl coA activated pyruvate carboxylase in gluconeogenesis
glutton
LDL; LDL oxidize atherosclerosis
Phenylalanine build up

44. How do you treat orotic aciduria? How does this work?
No
Oral uridine administration; provides nucleosides and provides feedback inhibition
Ile - Phe - Thr - Trp
Downs (21) - Edwards (18) - Pataus (13); also in increasing severity

45. What is the degradation product of Norepinephrine?
VMA
anabolic processes
Cancers (rapidly synthesizing); MTX (blocks dihyrdofolate reductase); phenytoin (prevent conjugase from breaking down polyglutamate form to monoglutamate form; reabsorption in jejunum is blocked by alcohol and OCPs; also blocked by sulfonamides
Carbomyl phosphate synthetase I turns N acetyl glutamate into carbomyl phosphate and then ornithine transcarbomyalse turns carbomyl phosphate into citrulline (using ornithine)

46. What kind of kinetics does alcohol dehydrogenase work under? What are the implications of this?
5' capping (with 7 methylguanosine) - 3' adenylation - splicing of introns; nucleus
When there is glucagon; protein kinase A activation and thus phosphorylation of the complex resulting in the activation of fructose 2 -6 BPase and no PFK2
Zero order; increased production of NADH no matter the amount of end products produced
NF2 - DiGeorge/Velocardiofacial syndromes

47. _____________ prevents fluidity of plasma membrane
enzymatic
high cholesterol
RER in nucleus (synthesize NTs)
Reserve supply of glucose in the fasting state - liver gives to the blood and skeletal muscle uses for itself

48. What is the findings in Fabrys disease?
Peripheral neuropathies of hands/feet; angiokeratomas between umbilicus and knees; cardiovascular/renal disease
Orotic acid in blood and urine (from pyrimidine synthesis) - decreased BUN - and symptoms of hyperammonemia
fastest sugar
both liver and muscle (unlike glucagon)

49. What are stable/quiescent cells? Which cells are stable?
paternal
They can enter G1 from G0 and replicate if they like; hepatocytes - lymphocytes - fibroblasts
Lyonization; nondisjunction in mitotic divisions during embryonic period
AR

50. What is carnitine deficiency and What does it result in?
aldose reductase
Cystic Fibrosis (CFTR gene)
Tryptophan; melatonin
Inability to transport LCFA into mitochondria results in their build up and causes weakness - hypotonia And HYPOKETOTIC HYPOGLYCEMIA

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USMLE Step 1 Biochemistry

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