Problem Based Multiple Choice Questions (Set-3)- Solved with complete discussion

31- A 47- year-old woman passes out at work and is brought to an emergency room. Her blood sugar is 34 mg/dl (normal >60 mg/dl). She is also tested for C-peptide, which is much higher than normal. What is the possible cause of this woman’s state?

a) Anorexia

b) High protein diet

c) Exogenous administration of Insulin

d) Insulin secreting Islet cell tumor

e) High glucose diet

32-A 24 -year old bride prepares for her wedding day. After her manicure and pedicure, her hairdresser uses rollers to create a new style for her hair. To create a “permanent wave”, the stylist then applies thioglycollate to break apart the-S-S- bonds in cystine units, reducing them to -SH groups. Which level of protein structure is most greatly affected by this treatment?

a) Primary structure

b) Secondary structure

c) Tertiary structure

d) Quaternary structure

e) Super secondary structure

33- Oral contraceptives are known to lower blood levels of folate and B6. Which of the following conversion processes would be directly affected?

a) Glutamate to N-Acetyl Glutamate by NAG synthase

b) Glutamate to Glutamine by Glutamine synthase

c) Glutamine to Glutamate by Glutaminase

d) Glutamate to Alpha-ketoglutarate by Glutamate dehydrogenase

e) Glutamate to GABA by Glutamate decarboxylase

34-In women taking thyroid hormone replacement pills, the dosage must be adjusted if they start taking birth control pills. Which one of the following best explains this situation?

a) Thyroid hormones block the action of estrogens, so the estrogen dose must be increased.

b) Estrogens block the action of thyroid hormones, so the dose of thyroid hormone must be increased.

c) Progestins block the action of thyroid hormone, so the dose of thyroid hormone must be increased.

d) Thyroid hormones stimulate the action of estrogens, so the estrogen dose must be decreased.

e) Estrogens stimulate the action of the thyroid hormone, so the dose of thyroid hormone must be decreased.

35- A 57-year-old alcoholic is transported to the emergency room after sustaining an injury in a motor vehicle accident. A comprehensive metabolic panel and a serum γ- glutamyl transferase (GGT) level are ordered. The GGT is shown to be dramatically elevated. Apart from its role as a marker of alcoholism, this enzyme is also important for which of the following digestive processes?

a) Recycling of bile salts

b) Absorption of carbohydrates

c) Digestion of triglycerides

d) Absorption of amino acids

e) Digestion of carbohydrates

36-A 64-year -old man complains of acute onset of unilateral eye pain and reduction in visual acuity. On physical examination, a conjunctival injection (redness of eye),a mild-dilated and non-reactive pupil is observed. Fundoscopic examination reveals the cupping of the optic disc. Recognizing the signs and symptoms of glaucoma, Acetazolamide medication is prescribed to reduce the production of aqueous fluid and lower the intraocular pressure. Acetazolamide is a non-competitive inhibitor of carbonic anhydrase, and therefore will cause ;

a) An increase in Km

b) A decrease in Km

c) An increase in Vmax

d) A decrease in Vmax

e) A decrease in both Km and Vmax

37-A 12- A year-old Asian girl presents with nausea, vomiting and non-localizing neurological signs. She has been using brightly colored facial cosmetics obtained from Abroad.

Which biochemical investigation should be appropriate?

a) Liver function tests

b) Brain scan

c) Blood protoporphyrin level

d) Estimation of zinc levels

e) Estimation of arsenate levels

38-A 43-year-old alcoholic male has been taking the drug Cimetidine for gastric reflux. His primary care physician warns that this is not a good idea given his poor liver functions, and therefore, his decreased ability to metabolize the drug by Glucuronidation. The Glucuronidation involves the addition of a glucose molecule that has been changed by which of the following processes to form glucuronic acid?

a) Oxidation

b) Reduction

c) Hydrolysis

d) Sulfation

e) Methylation

39-A 5 -year-old child presents with Hurler syndrome, which is characterized by dwarfism, hunchback, coarse facies, mental retardation, clouding of the cornea, and sensorineural deafness. The patient also has organomegaly due to the accumulation of which of the following? 

a) Glucocerebrosides

b) Sphingolipids

c) Heparan sulfate and dermatan sulfate

d) Glycogen

e) Galactose-I- Phosphate

40- As electrons are received and passed down the transport chain, the electron carriers are first reduced with the acceptance of the electron and then oxidized with loss of the electron. A patient poisoned by which of the following compounds has the most highly reduced state of most of the respiratory chain carriers?

a) Antimycin A

b) Rotenone

c) Carbon monoxide

d) Puromycin

e) Chloramphenicol

41- A 70-year-old female with Alzheimer’s dementia wanders away from her home. She gets lost and stays without water and food for 5 days. After 5 days of starvation, which of the following processes would provide the primary source of energy for the brain? 

a) Glycogenolysis

b) Lipolysis

c) Gluconeogenesis

d) Glycolysis

e) Ketosis

42-A 6 -month-old infant presents with stools that are pale-colored, bulky, and foul-smelling. Deep tendon reflexes are absent, his vision is poor, and he has difficulty in walking. He is diagnosed with Abetalipoproteinemia, caused due to an inability to synthesize lipoproteins containing apo B protein. Which of the following lipoprotein would be in a normal concentration in this disorder?

a) HDL

b) VLDL

c) LDL

d) IDL

e) Lp (a)

43-A 40-year- old male is brought to the intensive care unit after being hit by a car. He has severe internal bleeding and is transfused with multiple units of red blood cells to replace his lost blood. In order for hemoglobin to release oxygen to his tissues 2,3 bisphosphoglycerate is needed, which is synthesized in the glycolytic pathway, by RL shunt, a divergence of the glycolytic pathway, unique to red blood cells. How many ATP molecules are expected to be produced per molecule of glucose when RL shunt is on in the red blood cells?

a) 2 

b) 4

c) 38

d) 36

e) ZERO

44- A 27 –year-old female with epilepsy has been taking Phenytoin to control her seizures. She is pregnant and considering the risk of potential bleeding episodes in the infant, her physician is considering to change her medicine. What biochemical activity might be deficient in the baby if her medication is continued as such?

a) Hydroxylation of proline

b) Gamma carboxylation of glutamic acid residues

c) Glucuronidation of bilirubin

d) Oxidation of lysine

e) Oxidation of glutathione.

45-A 6-month-old infant is seen in the emergency room with a fractured rib and subdural hematoma.  His serum copper level is 4.5 nM/L (Normal range is 5-11 nM/L). Developmental delay is apparent. Which of the following enzyme activity is most affected corresponding to the symptoms?

a) Lysyl oxidase

b) Carboxy peptidase

c) Tyrosinase

d) Superoxide dismutase

e) Xanthine oxidase

Answers-

31-  d)-Insulin is synthesized in the Pre proinsulin form (Immature form of insulin)  that is cleaved to Proinsulin. Proinsulin is converted to a mature functional form of insulin by the removal of C-peptide. Insulin and C-peptide are synthesized in the equimolar concentrations. In conditions of dietary deficiency of carbohydrates or in anorexia (as mentioned in the study) the stimulus for high insulin release is not there hence the high C-peptide levels are not expected. If she had been self-administering pharmacological insulin, it would not have raised C-peptide levels. Under such conditions, the most likely answer is Insulin secreting Islet cell tumor. In this condition, there is a large excessive release of insulin to cause a similar elevation in C- peptide levels.

 32- c)-  Tertiary structure is the right answer. Tertiary structure is stabilized by covalent disulfide bonds as well as by hydrophobic interactions, electrostatic interactions, and hydrogen bonds. The primary structure is composed of covalent amide bonds. The secondary, super secondary and the quaternary structure are stabilized by non-covalent interactions.

 33- The right answer is e). Out of all the mentioned enzymes, Glutamate decarboxylase is the only enzyme that is B6 dependent, hence GABA formation would be the only process affected by a B6 deficiency.

N-Acetyl Glutamate by NAG synthase is required as a positive allosteric modifier by Carbamoyl phosphate synthetase-1 enzyme, the first enzyme of the urea cycle. Glutamine formation and hydrolysis are active processes taking place in the kidney, liver, and brain for detoxification of ammonia and for maintenance of acid-base balance, especially in the kidney. Glutamate dehydrogenase is exclusively present in the liver and can utilize either NAD⁺ or NADP⁺.

34-b)–Estrogens partially block the action of thyroid hormones, making them less effective

35- The right answer is d). Glutathione has an important role in the absorption of amino acids. This is through the Gamma-glutamyl cycle or the Meister cycle.

36- The right answer is d) A decrease in Vmax. Acetazolamide is a non-competitive inhibitor, hence the Km value will remain the same. Had it been a competitive inhibitor, km value would have increased, Vmax would have remained the same. Vmax does not increase in any type of inhibition. A decrease in both Km and Vmax is observed in uncompetitive inhibition.

37-c)  Many imported cosmetics contain lead. Very frequently children ingest these agents and develop lead poisoning. It is thus appropriate to measure the whole blood lead and erythrocyte protoporphyrin level since Lead inhibits the pathway of haem biosynthesis at the level of ALA dehydratase and Ferrochelatase.

38- The right answer is (a) Glucose is oxidized to  Glucuronic acid through the uronic acid pathway of glucose metabolism. Gluconic acid is formed by oxidation of aldehyde group of glucose, while Glucuronic acid is formed by the oxidation of the primary alcoholic group when the aldehyde group is shielded by UDP. When both aldehyde and alcoholic groups are oxidized it is Glucosaccharic acid.

In the process of conjugation, UDP Glucuronic acid acts as s donor of glucuronic acid for conjugation of xenobiotics as well as for the endogenous harmful compounds. The addition of glucuronic acid imparts polarity to the foreign substance to make it water-soluble and easily excretable.

39- The right answer is (c) In Hurler syndrome (Mucopolysaccharidosis ), there is a deficiency of α-L-Iduronidase enzyme. This enzyme removes Iduronic acid from the chain and then subsequently, the other residues are removed by the specific enzymes. In its deficiency degradation of dermatan sulfate and heparan sulfate are affected. These GAGs (Glycosaminoglycans) get accumulated in the tissues producing a variety of symptoms characteristic of this disease. These GAGs are also excreted excessively in the urine.

40- The right answer is (c)- Carbon monoxide. The electron transport chain contains three proton pumps linked by two mobile electron carriers. At each of these three sites (NADH-Q reductase, cytochrome c reductase, and cytochrome oxidase), the transfer of electrons down the chain powers the pumping of the protons across the inner mitochondrial membrane. The blockage of electron transfers by specific point inhibitors leads to a buildup of highly reduced carriers behind the block because of the inability to transfer electrons across the block. Antimycin A blocks the complex III, Rotenone blocks complex I and Carbon monoxide (As well as cyanide, azide, hydrogen sulfide) block complex IV. Therefore a carbon monoxide inhibition leads to a highly reduced state of all the carriers of the chain. Puromycin and Chloramphenicol are inhibitors of protein synthesis and have no direct effect upon the electron transport chain.

41- The right answer is (e). Glycogen stores are sufficient only for 24-48 hours, gluconeogenesis starts thereafter to provide glucose to brain cells. Fatty acids cannot cross the blood-brain barrier due to the presence of a long hydrophobic chain thus brain cells cannot use fatty acids as a source of energy. In the starvation state (3-5 days of fasting), gluconeogenesis decreases due to reduced availability of the precursors, ketone bodies are synthesized by the liver and are used by the peripheral cells and brain as an alternative source of energy.

42- The right answer is HDL, this is the only lipoprotein that does not contain Apo B protein, rest all of them have apo B protein, HDL has all the apoproteins (A, C, D, and E) and is a transporter of cholesterol from peripheral tissues back to the liver for degradation. It is antiatherogenic, that is why called,” Good cholesterol.” VLDL contains apo B100 and is a transporter of endogenous triglycerides from the liver to peripheral tissues. LDL and IDL are formed from VLDL, thus they essentially contain apo B, protein. LDL is a transporter of cholesterol from the liver to peripheral tissues, Excess of cholesterol is atherogenic, That is why LDL is called ” Bad cholesterol”. Lp (a) is attached with apo B-100 by disulfide linkage, it is atherogenic and its high concentration is associated with increased risk for myocardial infarction.

43- Answer- The right answer is (e) Zero ATPs. There are no mitochondria in red blood cells. The net energy yield per glucose molecule is 2 ATP. In total 4 ATP molecules are produced ( 2 ATP at the level of conversion of 1,3 bisphosphoglycerate to 3 Phosphoglycerate and the other 2 ATP at the level of conversion of Phosphoenolpyruvate to Pyruvate), these are produced by substrate-level phosphorylation reactions without involving the electron transport chain (since there are no mitochondria ). 2 ATPS are spent initially during priming reactions, first at the level of phosphorylation of Glucose (first reaction of glycolysis catalyzed by hexokinase/ glucokinase) and the second at the level of phosphorylation of fructose-6-P to form Fr 1,6 Bisphosphate (catalyzed by PFK-1, rate-limiting enzyme of glycolysis). Thus the net energy output per glucose molecule is 4-2= 2 ATP. When RL shunt operates in RBCs the first substrate-level phosphorylation (Conversion of 1,3 BPG to 3 PG) is bypassed, no ATP at this step. Thus the total energy output is 2 ATP and the net energy output considering the initial energy consumed in the priming reactions becomes 2-2= 0 ATP. 
This shunt has importance due to 2,3 bisphosphoglycerate that lowers the affinity of Hb for O2 (by maintaining it in the deoxy state) which is needed for unloading of O2 to peripheral tissues.

Figure- Showing RL shunt

44 – The right answer is (b) Gamma carboxylation of glutamic acid residues It is vitamin K deficiency induced by phenytoin. Hydroxylation of proline is brought about by vitamin C. Glucuronidation of bilirubin, oxidation of lysine and oxidation of glutathione is not related to phenytoin. Vitamin K is also required for bone formation. Vitamin K deficiency induced by phenytoin is also responsible for fetal malformation of bones called fetal hydantoin syndrome. The cause of hemorrhage in a newborn is not vitamin C deficiency but it is vitamin K deficiency since vitamin K acts as a coenzyme for carboxylation of glutamic acid residues of factor II, VII, IX, and X.

45- The right answer is -a) Lysyl oxidase. The child is suffering from Menke’s disease which is a copper deficiency disorder. Lysyl oxidase enzyme is a copper-dependent enzyme, required for the hydroxylation of lysine to hydroxylysine. Vitamin C is also required for this hydroxylation reaction. This is an important step in collagen synthesis. The signs of fractured rib and subdural hematoma are due to impaired collagen formation in copper deficiency.
Carboxy peptidase is a zinc-dependent enzyme required for the removal of amino acid from the carboxyl-terminal of a peptide during digestion.
Tyrosinase is an enzyme for melanin synthesis, that is also a copper-dependent enzyme, the activity of this enzyme is also grossly affected in Menke’s disease.
Super oxide dismutase is also a copper dependent natural antioxidant enzyme.
Xanthine oxidase is a metalloenzyme, requiring molybdenum for its activity, it is required for the conversion of hypoxanthine to xanthine and then to uric acid.
Under the present circumstances of copper deprivation, the enzyme activities of all the copper-dependent enzymes are affected, but in the given case, as per the signs and symptoms described the most appropriate answer is a) Lysyl oxidase.