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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 aa does tyrosine come from?
Structural (constitutively active)
Phenylaline
Cri du Chat
Sulfonamides - primaquine - dapsone - antiTB drugs - fava beans - LDL; RBCs

2. What causes liver damage in kwashiokor?
Oxaloacetate depleted for gluconeogenesis; excess NADH shunts oxaloactetate towards malate
Increased TG synthesis from increased Glycerol 3 P and abetalipoproteinemia
Neural tissue and adrenal medulla
2/3; LCAT (lecithin cholesterol acyltransferase)

3. What is the deficient enzyme in Metachromatic leukodystrophy disease? accumulated Substrate?
Arylsulfatase A; cerebroside sulfate
When water is pulled in; polyols (sugar alcohols); cataracts - retinopathy - peripheral neuropathy all seen in chronic hyperglycemia in diabetes
An AP (apyrimidinic) endonuclease to cut the DNA - exonuclease then removes and then filled and sealed by enzymes
CPSII

4. What is the primary source of nitrogen in the urea cycle
Glutamate
Cleft palate - Abnormal faces - thymic aplasia (t cell def) - Cardiac defects - hypocalcemia (secondary to hypoparathyroid); 3rd and 4th branchial pouch aberrant development
It inhibits conversion of DHF to THF by blocking dihyrdofolate reductase
Triacylglycerols in adipose tissue broken down into glycerol and free fatty acids; growth hormone and epinephrine

5. Which drug blocks the Na+K+ pump by blocking the K+ site?
2/3; LCAT (lecithin cholesterol acyltransferase)
Coenzyme A and lipoamides
Ouabain
Actin/myosin - MT - intermediate filaments

6. What vit def occur from antibiotic use?
HMP shunt (to NADPH) and glycogenolysis (as glucose 1 phosphate)
NADH - Acetyl CoA - ATP; NAD+ - ADP and Ca2+
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables
Biotin (vit B7) and Vit K

7. Which lysosomal storage diseases have enzymes that are def in cells that are neuronal mostly?
Neimann pick and tay sachs
ATP and dATP
Acidic
Down's Syndrome

8. What are the functions of zinc?
Respiratory burst - P450 - and glutathione reductase
(cofactor for 100+ enzymes!) needed for zinc fingers transcription motif - collagen synthesis - alcohol metabolism
TPP (thiamine pyrophosphate)
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

9. From What aa does dopa come from?
Arginine
N5N10 methylene THF
Tyrosine
Neurons and RBCs

10. Walk through the process that adipose tissue makes triacylglycerol.
Carbomyl phosphate synthetase I
can be d/t lyonization
Ehler Danlos; type III collagen def; joint dislocation - berry aneurysm - ecchymoses - organ rupture
Lipoprotein lipases take FA from VLDL and chylomicrons Which are then combined with glycerol 3 P (a carb intermediate) to make triacylglycerol

11. What is a nissl body?
Urine turns black on standing
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
Gets converted to pyruvate via the Cori cycle in the liver
RER in nucleus (synthesize NTs)

12. What is the degradation product of Norepinephrine?
Mental retardation - rocker bottom feet - micropthalmia - microcephaly - cleft lip/palate - holoprosoncephaly - polydactyly
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
Fabrys Disease and Hunters Syndrome
VMA

13. Findings for McArdle's disease? deficient enzyme?
Night blindness - dry skin
Lupus
High glycogen in muscle - but cannot break it down - leading to painful muscle cramps - myoglobinuria with strenuous exercise - no increase in lactic acid after exercise; skeletal muscle glycogen phophorylase
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

14. What two anti - neoplastic drugs work by blocking DNA and RNA polymerase?
Pataus
Hexosaminidase A; GM2 ganglioside
Carbomyl phosphate synthetase II
Dactinomycin - doxorubicin

15. Breast milk has a little of...
5'--> 3'; prokaryote only; for removing RNA primer and filling it with DNA
anabolic processes
Respiratory - seminal fluid - pancreatic - gallbladder; very thick without NaCl to pull water
Vit D and K

16. What converts fructose 2 -6 phosphate back to fructose 2 phosphate?
Fructose bisphosphatase 2
tumor suppressor genes (p53 - Rb - BAX)
Dactinomycin - doxorubicin
Inability to transport LCFA into mitochondria results in their build up and causes weakness - hypotonia And HYPOKETOTIC HYPOGLYCEMIA

17. why is there decreased immunity in kwashiokor and marasmus?
HMP shunt (to NADPH) and glycogenolysis (as glucose 1 phosphate)
Decreased phenylalanine - increase tyrosine
COP II
No protein!

18. What can cause an excees of vit A? What are the symptoms?
Tyrosine hydroxylase; tetrahydrobiopterin factor; NADP+
show disease
Iatrogenic - granulomatous disease (sarcoidosis d/t epitheloid macrophage release of calcitriol); hypercalcemia - hypercalciuria (renal calculi!) - loss of appetite - stupor
Consumption of bear liver - and overtreatment isoretinoin; constitutional symptoms and alopecia and skin changes

19. What is there on increased risk of with Downs?
Fructose bisphosphatase 2
All and Alzheimers
Steroid - cholesterol FA synthesis; HMP shunt
Yes! congenital simply means 'born with'- hereditary is familial

20. What does actin/myosin participate in?
Microvilli - muscle contraction - cytokinesis - adherens junctions
homocysteine transferase
Self mutilation - aggression - mental retardation - gout - hyperuricemia - choreathetosis
Tryptophan; niacin

21. What are the two disorders of galactose metabolism? Which is worse?
Lactose in mothers milk contains galactose
Galactokinase deficiency and classic galactosemia; classic galactosemia
Tyrosinase
Acetyl coA activated pyruvate carboxylase in gluconeogenesis

22. Which is the rate limiting step of the urea cycle?
Carbomyl phoshpate synthetase I
Fructose bisphosphatase 2 and Phosphofructokinase 2; phosphorylation by protein kinase A; inuslin/glucagon ratio
degradation
Hepatic fatty change

23. What are the types of lipids found in the plasma membrane?
Glutamate; glutamate decarboxylase; B6; gamma aminobutyrate
Phospholipids - sphingolipids - glycolipids
Gets converted to pyruvate via the Cori cycle in the liver
Also have fair skin - blond hair - blue eyes - eczema

24. What is needed after the glycosylase in base excision repair?
Lack of lactase (brush border enzyme) so can not break down lactose (a dissaccharide of galactose and glucose); African Americans and Asians
Methionine is only encoded by AUG
An AP (apyrimidinic) endonuclease to cut the DNA - exonuclease then removes and then filled and sealed by enzymes
TPP (from B1- pyrophosphate) - FAD (B2) - NADH2 (B3) - CoA (B5 - panothenate) - Lipoic Acid

25. What is calcitriol?
The active form of Vit D (1 -25 (OH)2 D3)
Following gastroenteritis and kwashiokor (protein deficiency)
Neimann pick and tay sachs
Peripheral neuropathies of hands/feet; angiokeratomas between umbilicus and knees; cardiovascular/renal disease

26. other than bone resorption to release Ca and phosphate - What is the ultimate point of increasing the serum Ca2+ and phosphate?
Lysine and leucine
To increase serum Ca and phosphate for bone deposition
3 hydrogen bonds; 2 hydrogen bonds
Bacterial synthesis in the intestine

27. How is CF diagnosed?
H1
Heteroplasmy
Ile - Phe - Thr - Trp
Increased concentration of Cl ions in sweat test

28. What is the deficient enzyme in Tay Sachs disease? accumulated Substrate?
Hexosaminidase A; GM2 ganglioside
Lime and CaCO3 to displace the niacin
acetyl coA
Edwards

29. What structures allows for degeneracy?
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)
TRNA wobble
Rb
Glutamate

30. What syndrome is characterized by hyperextensible skin and joints - and bleeding? What is the most common reason? What are other clinical symptoms?
Orotic acid in blood and urine (from pyrimidine synthesis) - decreased BUN - and symptoms of hyperammonemia
Telomerase; an RNA polymerase that uses a RNA template to add DNA to the end of shortening 3' ends
Ehler Danlos; type III collagen def; joint dislocation - berry aneurysm - ecchymoses - organ rupture
Schilling test

31. Where do aminotransferase reactions occur? and What vitamin cofactor do they require?
Ascites from liver damage
Bendy bones seen in children due to decreased Vit D; craniotabes - skeletal deformities - growth retardation - rosary
In hepatocytes; B6
Dopamine

32. Which antihelminthic drugs act on microtubules?
Mebendazole and thiabendazole
A fibrous protein (ie produced by fibroblasts); it has lysine and proline but nonglycosylated; arteries - lungs - elastic ligaments - vocal cords - ligamenta flava (connect vertebrae); broken down by elastase Which is inhibited by alpha 1 antitrypsin
Phenylaline
Its the ribose; its made from PRPP synthetase using ribose 5 P from the HMP shunt

33. What is the main clinical presentation of hartnup disease?
the RER
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
Pellagra
In hepatocytes; B6

34. What is the first enzyme step in glycogenolysis? What activates it ? inactivates?
Glycogen phosphorylase
Microcephaly - mental retardation - high pitched mewing/crying - epicanthal folds - cardiac abnormalities; microdeletion on short arm of chrom 5
Mental retardation - flat faces - simean crease - epicanthal folds - ASD - duodenal atresia causing Hirshsprungs disease - redundant skin at the nape of neck - slanted palpebral fissures - hypotonia
Glycogen phosphorylase; phosphorylation and dephosphorylation

35. What can cause deficiency in Vit K?
Myotonic Dystrophy (CTG) - Huntington (CAG) Friedricks Ataxia (GAA) - Fragile (CGG)
Neimann pick and tay sachs
Vit D levels
Prolonged Ab use

36. What disease is associated with chromosome 4?
Hungtington
Differentiation of cells - vision in reduced light (retinol makes up rhodopsin cells) - antioxidant
MT!
Glycine; heme

37. AD diseases are usually...
HMP shunt (to NADPH) and glycogenolysis (as glucose 1 phosphate)
pleiotropic
AST (mit damage) - and GGT (d/t SER hyperplasia d/t induction of p450 generation)
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

38. What is elastin? How is it different from collagen? Where is it found? How is it broken down?
Coarse facial features - psychomotor retardation - clouded corneas - restricted joint movement - high plasma levels of lysosomal enzymes
A fibrous protein (ie produced by fibroblasts); it has lysine and proline but nonglycosylated; arteries - lungs - elastic ligaments - vocal cords - ligamenta flava (connect vertebrae); broken down by elastase Which is inhibited by alpha 1 antitrypsin
are AR
Bacteria produce lactate acid and gases from lactose and produce acidic diarrhea and bloating

39. What is the presentation for cystinuria?
Cystine kidney stones (cystine staghorn calculi)
AD; disorder of blood vessels that results in telangietasias - skin discolorations - and AVMs (arteriovenous malformations)
enzymatic
Following gastroenteritis and kwashiokor (protein deficiency)

40. What dictates whether a cell will regenerate or repair after inflammation?
Downs
N linked oligosaccharides
All clathrin
1) if it is stable/labile or permanent 2) intact BM and 3) intact ECM

41. What are the key intermediates in the TCA?
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
Citrate - Isocitrate - alpha ketoglutarate - Succinyl CoA - Succinate - Fumarate - Malate - Oxaloacetate
Limit dextrans
Homocysteine; homocysteine methyltransferase with B12 and tetrahydrofolate

42. What control over pyruvate kinase does insulin have?
Citrate is positive on it - AMP and fructose 2 -6 bisphosphate is negative on it; gluconeogenesis
Coarse facial features - psychomotor retardation - clouded corneas - restricted joint movement - high plasma levels of lysosomal enzymes
It induces its action!
They use the H2O2 produced by some bacteria to produce bleach with myeloperoxidase Which they still have

43. What is the rate determining enzyme in glycogenolysis?
Niacin/Nicotinic acid; NAD+ and NADH used in redox reactions - and catabolic and anabolic reactions; B6; tryptophan
Mitochondria - archaebacteria - mycoplasma - and some yeasts
Carbamoyl phosphate to orotic acid
Glycogen phosphorylase

44. All the glycogen storage diseases...
Cytoplasmic side; active
Later
are AR
Galactokinase; galactose appears in blood and urine - sometimes infantile cataracts may present as failure to track objects or lack of a social smile

45. What dose a deficiency in Vit B12 cause? What causes it?
in hepatocytes
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
HnRNA; mRNA
Cortisol; inhibits insulin action

46. Where is antioxidant activity of Vit E most important?
Neurons and RBCs
cell cycle events
N5N10 methylene THF
1) if it is stable/labile or permanent 2) intact BM and 3) intact ECM

47. Is heterochromatic more accessible or less accessible to TF? euchromatin?
Less; more
Fructose gets phosphorylated into fructose 1- P by fructokinase and then fructose 1P gets metabolized into DHAP or glyceraldehyde by aldolase B
Alcohol dehydrogenase; cytosol; mitochondria; NAD+
Downs (21) - Edwards (18) - Pataus (13); also in increasing severity

48. why does galactose def present in newborns>
Lactose in mothers milk contains galactose
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
2 NADH - 2 ATP - 2 pyruvate - 2H+ - 2H20
Excess ATP and dATP has negative feedback inhibition on ribonucleotide reductase Which results in decreased DNA synthesis and thus decreased lymphocyte count

49. What converts dopa into melanin?
Isoleucine - valine - leucine
Tyrosinase
False; GTP hydrolysis
2 -4 dinitrophenol - aspirin - and thermogenin (in brown fat)

50. What is uniparental disomy? give an example of a disease it can cause.
Later
When child gets two chromosomes from one parent; imprinting dx
Developmental delay - gargoylism - airway obstruction (d/t short neck) - corneal clouding - CAD - hepatosplenomegaly
Fe2+; Fe3+

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USMLE Step 1 Biochemistry

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