Usmle Biochemistry Qbank 4

Front 1

List out metabolic processes that occur in the mitochondria

Back 1

beta oxidation of FA, ketogenesis, TCA cycle, parts of the urea cycle (CPS I and OTC) and pyruvate carboxylation; all reactions of pentose phosphate pathway

Front 2

Catecholamines and glucagon stimulates what metabolic pathway?

Back 2

Glycogenolysis

Front 3

Glycogenolysis is stimulated with what molecules binding to cell receptors?

Back 3

Catecholamines (NE and Epi) and glucagon

Front 4

Which molecule increases gluconeogenesis in the liver when it binds to cell receptors?

Back 4

Cortisol

Front 5

How do you distinguish initial presentation of fructose intolerance vs. classic galactosemia

Back 5

Both present similarly with vomiting, hepatomegaly, and jaundice.

However, classic galactosemia occurs soon after breastfeeding is initiated (galactose in breast milk), whereas fructose intolerance occurs soon after introduction of fructose into the diet (after weening)

Front 6

Sickle cell dz has what kind of inheritance pattern

Back 6

AR

Front 7

How can one identify sickle cell trait on lab analysis?

Back 7

Electrophoresis - HbS moves slowly compared to normal Hb, due to loss of the negatively charged amino acid glutamate that is switched to valine

Front 8

Write out the G-protein dependent phopholipase C pathway.

Back 8

Refer to image

Front 9

Branched-chain alpha ketoacid DH, pyruvate DH, and alpha-KG DH all require which cofactors?

Back 9

Thiamine pyrophosphate, Lipoic Acid, Coenzyme A, FAD, NAD

"Tender Loving Care For Nancy"

Front 10

Maple Syrup Urine Dz Clinical presentation and pathogenesis

Back 10

Blocked degradation of branched amino acids (Ile, Leu, Val - I love Vermont maple syrup from branched maple trees) due to decreased alpha-ketoacid DH.

Sx: severe CNS defects, MR, death

Front 11

What is pyridoxine involved in?

What is it used to treat?

Back 11

Pyridoxine (Vit B6) as pyridoxal phosphate is involved in the transamination and decarboxylation steps in AA metabolism as well as heme and NT synthesis.

Pyridoxine supplementaiton is used in the tx of sideroblastic anemia and hyperhomocysteinemia.

Front 12

A single AA residue replacement in the Hb beta subunit results in poor ionic interaction with 2,3-bisphosphoglycerate.

The resultant Hb has the properties of which type of Hb?

Back 12

HbF

Normally, 2,3-DPG forms ionic bonds with the two beta subunits of HbA in the tissues after Hb has been deoxygenated.

Fetal Hb binds O2 with a higher affinity due to its inability to interact with 2,3-DPG. Ultimately, the fetal Hb must be able to extract O2 from maternal Hb in the placenta.

Front 13

Describe regulation of the lac operon.

Back 13

1) negatively by binding of the repressor protein to the operator locus
2) positively by cAMP-CAP binding upstream from the promoter region.

Repressor proteins are constitutively expressed. Culture of E.coli in latose containing media causes a conformational change in the repressor protein, preventing its attachment to the operator region and increasing transcription of the lac operon structural genes.

Front 14

Hartnup dz: discuss pathophys and tx

Back 14

defective neutral amino acid transporter on renal and intestinal epithelial cells

Causes tryptophan excretion in urine and decreased absorption in gut (loose stools)

can be caused by vitamin b3 (niacin): Hartnup dz can result in niacin deficiency due to an excess loss of dietary tryptophan (remember that niacin is synthesized from tryptophan and that tryptophan is an essential AA.

PELLAGRA! -

Front 15

Discuss the mechanism of streptomycin

Back 15

Streptomycin inhibits the initiation of protein synthesis by binding to and distorting the structure of the prokaryotic 30S ribosomal subunit.

Front 16

Discuss mechanism of tetracycline.

Back 16

Interferes with binding of aminoacty-tRNA to the A site

Front 17

How do you distinguish clinically btw OTC and CPS I deficiency?

Back 17

Both OTC and CPS I deficiency result in increased ammonia levels. However, increased urine orotic excretion is only observed in OTC deficiency (accumulated carbamoyl phosphate is converted into orotic acid)

Q: What regulates CPS I?

Hint 17

N-acetylglutamate

Front 18

How are very long chain FA or FA with branch points at odd-numbered carbons oxidized?

What cellular structure is required for oxidation of these FA?

Sx?

Back 18

Special form of beta oxidation (for very long chain fatty acids) or Alpha Oxidation (branched chain fatty acids such as phytanic acid)!

Peroxisomes are required to break these down. Dz results in neurologic deficits from improper CNS myelination.

Front 19

14 yo female evaluated for neuro sx. at 10 percentile for weight and height. Lab reveals megaloblastic anemia with very high urine orotic acid secretion.

What supplementation would benefit this pt?

Back 19

Uridine

A pt with orotic aciduria (impaired e novo pyrimidine synthesis) will present with hypochromic megaloblastic anemia, neurologic abnormalities, growth retardation and excretion of orotic acid int he urine.

Uridine supplementation improves sx by inhibiting carbamoyl synthetase II

Front 20

When prescribing isoniazid, you should also prescribe what?

Back 20

Pyridoxine (Vit B6) supplementation

Front 21

Ascorbic acid fxn

Back 21

required for hydroxylation of proline and lysine residues in collagen synthesis; therefore, plays an important role in connective tissue maintenance and wound healing

Front 22

Niacin is an essential component of what coenzymes?

What AA is it synthesized from?

Deficiency results in what condition?

Back 22

NAD and NADP.

Tryptophan

Pellagra: dermatitis, diarrhea, and dementia (3Ds of Vit B3)

Front 23

Describe the sx of homocystinuria.

What supplementation may help with this condition?

Back 23

Cystathionine synthetase deficiency

Thromboembolic episodes involving both large and small vessles, especially those o the brain; Marfan syndrome like sx (elongated limbs, arachnodactyly, and scoliosis)

about 50% of pts respond dramatically to pyridoxine (Vit B6) supplementation (only tx at this time)

Front 24

homeless guy with increased fatigability and exertional dyspnea. Significant lower extremity edema and decreased sensation over ankles and feet. Cardiac dilation and increased CO noted.

What nutrient deficiency?

Back 24

Thiamine (Vit B1) deficiency - wet beriberi

Ber1Ber1

Front 25

Polyneuritis and symmetric muscle wasting. What nutrient deficiency?

Back 25

Vit B1 - Wet Ber1Ber1

Front 26

Hemorrhages, bleeding into joint spaces, gingival swelling, weakened immune system

Back 26

Vit C deficiency - scurvy

Front 27

Vit C deficiency - scurvy sx

Back 27

Hemorrhages, bleeding into joint spaces, gingival swelling, weakened immune system

Front 28

Pernicious anemia sx

Back 28

classically in mentally slow woman of northern European descent who is "lemon-colored" (anemic and icteric), has a smooth, shiny tongue indicative of atrophic glossitis, and demonstrates a shuffling broad-based gait.

Front 29

Increased HbF, HBA2, and target cells

Back 29

beta thalessemia

Q: What process is defective in thalassemias?

Hint 29

mRNA processing

Front 30

Intermediate source of nitrogen for urea in the urea cycle?

Back 30

Aspartate

(nitrogen atoms are also derived from NH3)

Front 31

Which AA transports ammonia from the peripheral tissues to the kidney?

Back 31

Glutamine

in the nephron, the amide's nitrogen is hydrolyzed by the enzyme glutaminase, thereby producing glutamate and free ammonium ion. The ammonium ion can then be excreted int he urine.

Front 32

Three prominent features of Marfans

Back 32

Extremities: long thin extremities, loose joints, and long fingers (arachnodactyly)

Ocular abnormalities such as dislocation of lens

Cardiovascular abnormalities including ascending aortic aneurysm, aortic dissection, and mitral valve prolapse.

Front 33

Which cells cannot use ketone bodies for energy?

Back 33

Erythrocytes and other cells lacking mitochondria. Also hepatocytes bc they lack a required enzyme

Front 34

Alanine is given to volunteers. Before conversion to glucose, the alanine transfers its amino group to _____.

Back 34

alpha-KG

Amino groups are released during the metabolism of AA. These amino groups are primarily transferred to alpha-KG to form glutamate, the primary carrier of ammonia from the liver. Glutamate is later excreted by the renal nephron as either free ion or urea.

Front 35

Homeobox (Hox) genes code for what protein type?

Back 35

Transcription regulators which alter the expression of genes involved in morphogenesis - proper formation and placement of tissues, organs and structural elements of the body.

Front 36

Defective ubiquination can result in what diseases?

Back 36

Alzheimer's and Parkinson's - failure of system to properly degrade abnormal proteins causes protein misfolding, aggregation, and eventual obstruction of intracellular molecular traffic, leading to cell death.

Front 37

Explain why wrinkles appear with age.

Back 37

Decreased collagen fibril production

Front 38

urine turns black upon standing

Back 38

alkaptonuria or acute intermittent porphyria

Front 39

How does high glucose infusion protect against acute intermittent porphyria?

Back 39

Glucose loading decreases porphyrin synthesis by repressing ALA synthase activity, thus alleviating the abdominal pain and neuropsychiatric manifestations of acute intermittent porphyria (AIP)

Front 40

Describe the pathway for dietary fructose to participate in glycogen synthesis

Back 40

see image

Front 41

Vit A overuse sx

Back 41

intracranial hypertension, skin changes and hepatosplenomegaly

Front 42

Radiation (chemotherapy) induces what type of DNA damage?

Alkylating agents?
UV radiation?

Back 42

DNA double strand fractures and the formation of oxygen free radicals

DNA cross-linking
pyrimidine-pyrimidine dimers (thymine dimers)

Front 43

Describe hemoglobin composition of fetal hemoglobin and adult hemoglobin.

Back 43

HbF dominates in newborns. It consists of two alpha and two gamma protein subunits, has a high affinity for O2 and is produced during the final seven months of gestation.

Switching to HbA (alpha 2 beta 2) occurs during the first six months of life

Front 44

A point mutation is found in the beta globin gene, which results in the replacement of G by C in the beta globin mRNA molecule three bases upstream from the AUG codon.

What process is most likely impaired?

Back 44

Initiation of translocation (mRNA binding to ribosomes)

This is the Kozak sequence which plays a role in the initiation of translation. A mutation three bases upstream from the start codon (AUG) in this sequence is associated with thalassemia intermedia.

Q: Thalassemia intermedia results in what kind of anemia?

Hint 44

hypochromic, microcytic anemia

Front 45

Why are patients with thalassemias asymptomatic at birth?

Back 45

Due to the presence of gamma globins in HbF. Switching to HbA production and the cessation of gamma globin synthesis precipitates the sx of thalassemia

Front 46

See image

Back 46

E

Front 47

How do you treat cyanide poisoning?

How does cyanide poisoning present?

Back 47

Nitrites (ie amyl nitrite) --> indues formation of methemoglobin, which has a high affinity for CN and keeps it from the blood and away from mitochondrial enzymes

- rapidly developing cutaneous flushing, tachypnea, H/A, tachycardia, often accompanied by N/V, confusion, and weakness. Respiratory distress and cardiac dysfunction may follow (mimic serotonin syndrome??)

Front 48

Pt with Ehlers-Danlos. Histologic evaluation with EM reveals collagen fibrils that are abnormally thin and irregular. Biochemical evaluation reveals abundant terminal disulfide bonds.

Which stage of collagen synthesis is most likely impaired in this patient?

Back 48

Extracellular Cleavage

Front 49

Pt develops a large bruise over the chest after MVA. Several days later, the bruise has a greenish color.

Which enzyme has created this color?

Back 49

Heme oxygenase

Front 50

Low activity of G6PD (resulting in hemolysis).

Deficiency of what RBC enzyme has the same pathophysiology as this condition?

Back 50

Glutathione reductase

- G6PD deficiency is a defect in the HMP shunt that impairs glutathione reduction due to failure to produce NADPH. Glutathione reductase deficiency causes a similar clinical picture and is pathophysiologically similar to G6PD deficiency.

Front 51

Functions of transketolase and transaldolase

Back 51

catalyze the non-oxidative reactions of the HMP shunt and are primarily designed to generate ribose 5-phosphate from fructose 6-phosphate and glyceraldehyde 3-phosphate or the reverse.

Front 52

Which vitamin supplementation can benefit a measles patient?

Back 52

Vitamin A

Front 53

see image

Back 53

A

Sigmoidal shape of hemoglobin oxygen-dissociation curve results from the tetrameric nature of hemoglobin and how after binding to one molecule,t he affinity of other heme molecules for oxygen increases (heme-heme interaction)

On the other hand, myoglobin has only one heme molecule, so its interaction with O2 is simpler. Myoglobin has a much higher affinity for O2 and does not experience heme-heme interactions --> therefore, its O2 dissociation curve is hyperbolic.

Front 54

List causes of splenic hypertrophy

Back 54

1) Intracellular substance accumulation (as in glycogen storage dz)
2) Passive congestion - portal hypertension, venous thrombosis and CHF
3) Inflammatory infiltration - spleen plays an important role in fighting infectious pathogens and thus, many infections lead to enlargement due to proliferation of lymphoid tissue.
4) Work hypertrophy - hemolytic anemia - spleen must work harder to remove deformed erythrocytes from circulation
5) Neoplasm - disorders such as leukemia and lymphoma due to neoplastic proliferation of lymphoid tissue within the spleen

Front 55

Lactic acidosis may be explained by low activity of what enzyme? High activity of what enzyme?

Back 55

Low activity of pyruvate DH or high activity of Lactate DH

Front 56

What is the principal stabilizing force for secondary structure of proteins?

tertiary structure?

Back 56

- H bonds are the principal stabilizing force for secondary structure.

- ionic bonds, hydrophobic interactions, H bonds, and disulfide bonds are all implicated in tertiary structure.

Front 57

An infant has impaired transport of ornithine from the cytosol to the mitochondria. Restriction of what type of energy source can improve this pt's condition (fatty acids, protein, cholesterol, monosaccharides, disaccharides, purines)?

Back 57

Protein - Ornithine transport into mitochondria is essential for urea formation, as ornithine is needed to combine with carbamoyl phosphate within the mitochondria to form citrulline in the second step of the urea cycle.

Front 58

End product of sucrose metabolism is ______.

Back 58

Fructose

Lens has significant levels of sorbitol DH, which become overwhelmed in the setting of hypoglycemia.

Other tissues, such as the retina, renal papilla, and Schwann cells, have much less sorbitol DH activity.

Front 59

Know the signaling pathways of endocrine hormones:

cAMP, cGMP, IP3, Steroid (cytosolic and nuclear), Intrinsic tyrosine (MAP kinase), receptor associated tyrosine kinase (JAK/STAT pathway)

Back 59

see image

Front 60

Explain the mechanism of lactic acidosis in patients with septic shock

Back 60

This occurs due to tissue hypoxia, which results in impaired OxPhos and the shunting of pyruvate to lactate following glycolysis.

Hepatic hypoperfusion also contributes to the buildup of lactic acid, as the liver is the primary site of lactate clearance.

Front 61

An elderly patient with dementia and/or hemiparesis develops lower lung pneumonia.

What is the most common cause?

Back 61

Aspiration pneumonia secondary to dysphagia due to swallowing muscle dysfunction.

- altered consciousness impairing cough reflex and glottic closure (dementia and drug intox)
- dysphagia due to neurologic deficits (stroke)
- Upper GI tract disorders (GERD)
- mechanical compromise of aspiration defenses (eg nasogastric and endotracheal tubes)
- protracted vomiting
- large volume tube feedings in recumbent position

Q: what are the dominant causes of aspiration pneumonia?

Hint 61

Anaerobic bacteria - Peptostreptococcus, Bacteroides, Prevotella, and Fusobacterium