Carbohydrate metabolism- Problem based MCQs

1) A 30-year-old man has been fasting for religious reasons for several days.  His brain has reduced its need for glucose by using which of the following substances as an alternative source of energy?

A.Fatty acids

B.Beta-hydroxybutyrate

C.Glycerol

D.Beta carotene

E. Alanine

2) A 7-year-old girl is brought to the emergency department by her parents with complaints of severe polyuria and polydipsia. Laboratory examination reveals ketones in her urine. Which of the following is the most likely source of ketones?

A.Fatty acid breakdown

B. Protein breakdown

C.Glycogenolysis

D.Gluconeogenesis

E. Side chain of cholesterol

3) A breast-fed infant began to vomit frequently and lose weight. Several days later she developed jaundice, hepatomegaly, and bilateral cataract. What is the possible cause of these symptoms? 

A. Galactosemia

B.Von-Gierke’s disease

C.Juvenile diabetes Mellitus

D.Hereditary fructose intolerance

E. Gaucher disease

4) The major metabolic product produced under normal circumstances by erythrocytes and by muscle cells during intense exercise is recycled through the liver in the Cori cycle. The metabolite is-

A.Oxaloacetate

B.Alanine

C.Glycerol

D.Lactate

E. NADH

5) A 3-month-old infant presents with hepatosplenomegaly and failure to thrive. A liver biopsy reveals glycogen with an abnormal, amylopectins like structure with long outer chains and missing branches.  Which of the following enzymes would most likely be deficient?

A.Alpha-Amylase

B. Branching enzyme

C. Debranching enzyme

D.Glycogen phosphorylase      

E. Glucose-6-phosphatase        

6) Prior to a race, many marathon runners will try to increase their glycogen concentrations by loading up with foods with high starch content, such as pasta. Alpha-amylase secreted by the pancreas will digest the starch into which of the following major products?

A.Amylose, amylopectin, and maltose

B. Glucose, galactose, and fructose

C.Glucose, sucrose, and maltotriose

D.Limit dextrins, maltose, and maltotriose

E.Maltose, glucose, and fructose

7) Which of the following substrates cannot contribute to net Gluconeogenesis in the mammalian liver?

A.Alanine

B.Glutamate

C.Palmitate

D.Pyruvate

E.Odd chain fatty acids

8) Which of the following complications is less likely to occur in type II diabetics, as opposed to type I diabetics?

A. Retinopathy

B. Weight gain

C. Cardiovascular disease

D. Hypoglycemic coma

E. Non ketotic hyperosmolar coma

9) Familial fructokinase deficiency causes no symptoms because

A. Hexokinase can phosphorylate fructose

B. Liver Aldolase can metabolize it

C. Excess fructose does not escape into the urine

D. Excess fructose is excreted through feces

E. Excess fructose is converted to glucose

10) Which of the following generates free glucose during the enzymatic breakdown of glycogen in skeletal muscles?

A. Phosphorylase

B. α-1-6-amyloglucosidase

C. Debranching enzyme

D.Glucose-6-phosphatase

E. Alpha amylase

1)   The answer is –B- Beta hydroxy Butyrate, a ketone body. Ketone bodies serve as an alternative fuel for the brain during prolonged fasting or starvation. Fatty acids due to the long hydrophobic chains cannot cross the blood-brain barrier. Glycerol is a substrate of gluconeogenesis. In fact, during prolonged fasting, this is the only substrate left to provide glucose through the pathway of gluconeogenesis. It can also be oxidized through glycolysis upon phosphorylation. Beta carotene is a provitamin; it is not a source of energy. Alanine is a transporter of the amino group of amino acids from the muscle (glucose-alanine cycle), but it cannot be used as an alternative source of energy.

 2)   The answer is –A. Fatty acid break down provides Acetyl co A that serves as a precursor for ketone bodies. In Diabetes Mellitus glucose utilization is impaired due to absolute or relative insulin deficiency. Fatty acid breakdown occurs to provide energy and the resultant excessive Acetyl co A enters the pathway of ketogenesis. Protein breakdown provides amino acids, 6 amino acids are ketogenic, while 14 are glucogenic. Hence protein breakdown contributes only a little towards the formation of Acetyl co A. The major contribution is through a fatty acid breakdown. Glycogenolysis and Gluconeogenesis produce glucose only. The sidechain of cholesterol provides propionyl co A which is a glucogenic component; it is converted to succinyl co A to gain entry into the TCA cycle.

 3)   The answer is –A. Galactosemia. The clinical manifestations are typical of classical Galactosemia. Bilateral cataract rules out the possibility of Von Gierke’s disease and hereditary fructose intolerance, although other symptoms are there in both these diseases. In juvenile diabetes mellitus, jaundice and hepatomegaly are not observed. In Gaucher disease, hepatomegaly is observed but cataract is never there.

 4)   The answer is-D- Lactate, the end product of glycolysis in erythrocytes and during intense exercise in skeletal muscle, is mobilized through the Cori cycle to the liver to provide glucose by the process of gluconeogenesis. (Erythrocytes lack mitochondria so the end product of glycolysis is always lactate.  The mode of glycolysis during intense exercise is anaerobic; hence lactate is formed as a result of glycolysis.

Alanine is transported to the liver through the Glucose Alanine cycle. Glycerol is also similarly transported but not from the erythrocytes or skeletal muscles, rather from the adipose tissues. Glycerol is a waste product in adipose tissues since without phosphorylation it cannot be utilized and the phosphorylating enzyme glycerol kinase is absent in adipose tissues.

 NADH produced at the step of glyceraldehyde dehydrogenase step is regenerated in the oxidized form NAD⁺ by reduction of pyruvate to lactate. These two reactions are coupled to have a continuous supply of NAD⁺.

 5)   The answer is-B- Branching enzyme. During the process of glycogen synthesis, branching enzyme creates branch points and further elongation is carried out by Glycogen synthase. In its deficiency stored glycogen is abnormal in chemistry, in the form of long polysaccharide chains with few branch points, resembling the structure of Amylopectin, thus this defect is also called Amylopectinosis. Alpha-Amylase is an enzyme for digestion of starch and glycogen. Debranching enzyme deficiency results in the accumulation of abnormal glycogen, There is an inability to remove the branch points, the resultant structure resembles Limit dextrin, and thus it is also called Limit dextrinosis. Glucose-6-phosphatase deficiency is observed in Von-Gierke’s diseases, a type 1 glycogen storage disease, the stored glycogen is always normal in chemistry.

 6)   The answer is-D.  The hydrolysis of starch is catalyzed by salivary and pancreatic amylases, which catalyze random hydrolysis of alpha (1- 4) glycoside bonds, yielding dextrins, and further hydrolysis yields a mixture of glucose, maltose, isomaltose (from the branch points in amylopectin) and maltotriose. Sucrose, galactose, and fructose are not components of starch.

 7)   The answer is- C. Palmitate, a fatty acid with 16 carbon atoms, is not a substrate of gluconeogenesis. Even chain fatty acids, predominantly present in our body, yield Acetyl co A upon oxidation, which can not contribute towards gluconeogenesis.  The Pyruvate to Acetyl co A conversion is irreversible and moreover both of the carbon atoms of Acetyl co A are lost in the TCA cycle in the form of CO2. Odd chain fatty acids do act as substrates of gluconeogenesis since propionyl co A the product of their oxidation can enter the TCA cycle through the formation of Succinyl co A, hence can contribute towards Glucose production. Alanine, pyruvate, and glutamine are glucogenic.

 8)   The answer is-D- Hypoglycemic coma occurs as a result of insulin overdosage in Type 1diabetes Mellitus.  It is not observed in Type 2 diabetes. Weight gain can occur in both types, it is the result of treatment with insulin or certain hypoglycemic drugs. Non-ketotic hyperosmolar coma is a frequent complication of coma, especially in the elderly group.

 9)   The answer is-A Hexokinase is a non-specific enzyme, it can phosphorylate fructose as well as other sugars but it has high km (low affinity) for fructose.  Glucose is the true substrate for this enzyme. Fructose-6-phosphate the end product of Hexokinase reaction can enter the glycolytic pathway to be utilized further, so it does not accumulate to produce the toxic effects. Liver Aldolase (Aldolase B) cleaves Fructose-1-P only, the product of fructokinase catalyzed reaction.  Aldolase A, present in all the cells of the body cleaves Fructose 1, 6 bisphosphate, the product of PFK-1 catalyzed reaction of glycolysis. Fructose to glucose conversion takes place only in the phosphorylated form.

 10) The answer is-B- Free glucose is released by the action of the α-1-6-amyloglucosidase enzyme, a component of the debranching enzyme. The debranching enzyme has two components. α-[1 4] to α-[1 4] Glucan transferase and α-1-6-amyloglucosidase.Glucan transferase shifts the trisaccharide on a branch bound by α-[1- 4] linkage to the straight chain and joins by α-[1 4] linkage. The exposed branch point is hydrolyzed by the α-1-6-amyloglucosidase enzyme. Both components are present on the same polypeptide chain. Glucose-6- phosphatase does produce free glucose but it is absent in skeletal muscles. Alpha-amylase is a digestive enzyme; it has no role in glycogen degradation in the muscle.