Glycogen phosphorylase is activated during fight-or-flight response primarily through which mechanism?
Answer: B
During the fight-or-flight response, epinephrine activates adenylyl cyclase leading to increased cAMP and PKA activation. PKA phosphorylates and activates glycogen phosphorylase, promoting glycogen breakdown for rapid glucose availability.
Q.22Medium
In the glycolytic pathway, the enzyme that catalyzes substrate-level phosphorylation in the ATP-generating step is:
Answer: D
Both phosphoglycerate kinase (1,3-bisphosphoglycerate → 3-phosphoglycerate) and pyruvate kinase (phosphoenolpyruvate → pyruvate) catalyze substrate-level phosphorylation in glycolysis, directly generating ATP.
Q.23Medium
A patient with type 2 diabetes shows impaired glucose uptake in muscle tissue. Which GLUT isoform is primarily defective?
Answer: D
GLUT4 is the insulin-dependent glucose transporter found primarily in skeletal muscle and adipose tissue. Impaired GLUT4 translocation or function is a hallmark of type 2 diabetes, leading to reduced muscle glucose uptake.
Q.24Medium
Which of the following statements about branching enzyme (α-1,6-glucosidase) is correct?
Answer: D
The branching enzyme (amylo-1,6-transglucosidase) transfers segments of α-1,4-linked glucose to create α-1,6-branch points, making glycogen more soluble and accessible. Its deficiency causes Andersen disease with abnormal glycogen accumulation.
Q.25Medium
In the Rapoport-Luebering shunt of RBCs, 2,3-bisphosphoglycerate (2,3-BPG) is produced from which glycolytic intermediate?
Answer: B
2,3-BPG is synthesized from 1,3-bisphosphoglycerate by bisphosphoglycerate mutase. 2,3-BPG binds to hemoglobin, decreasing its oxygen affinity, facilitating oxygen release to tissues. This shunt allows RBCs to regulate oxygen delivery without producing ATP.
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Q.26Medium
During fasting state lasting 24-48 hours, which substrate becomes the predominant fuel source for the brain after glucose stores are depleted?
Answer: B
During prolonged fasting, ketone bodies produced from hepatic fatty acid oxidation become the preferred fuel for the brain (up to 60% of energy needs), reducing the requirement for gluconeogenesis and preserving muscle protein.
Q.27Medium
Which enzyme is inhibited by feedback inhibition from glucose-6-phosphate and is thus rate-limiting for glucose entry in liver?
Answer: B
Hexokinase is inhibited by its product glucose-6-phosphate, making it rate-limiting. Glucokinase, found in liver, is NOT inhibited by G6P, allowing continuous glucose phosphorylation in fed state. This differential regulation is crucial for glucose sensing.
Q.28Medium
In McArdle disease (GSD Type V), the enzyme deficient is glycogen phosphorylase. Which of the following is the most likely clinical presentation during exercise?
Answer: B
McArdle disease involves glycogen phosphorylase deficiency in muscle, preventing glycogen breakdown during exercise. This causes exercise intolerance with muscle pain, cramps, fatigue, and myoglobinuria. The 'second wind' phenomenon (improved symptoms after 10 minutes as blood glucose increases) is characteristic.
Q.29Medium
The oxidative phase of the pentose phosphate pathway produces NADPH. Which of the following is the primary biosynthetic use of NADPH?
Answer: B
NADPH from the pentose phosphate pathway serves as the primary reducing agent for biosynthetic reactions, especially fatty acid and cholesterol synthesis. It is also crucial for antioxidant defense (glutathione reduction) and biosynthesis of nucleotides.
Q.30Medium
Which statement best describes the relationship between glycogen structure and its metabolic advantages?
Answer: B
Glycogen's highly branched structure with α-1,6-branch points every 8-12 glucose residues increases its solubility, provides multiple substrate sites for glycogen phosphorylase simultaneously, and enables rapid glucose mobilization during energy demands. This is metabolically superior to linear polymers.
Q.31Medium
In the citric acid cycle, which carbohydrate-derived molecule enters directly after acetyl-CoA formation?
Answer: D
Acetyl-CoA condenses with oxaloacetate to form citrate, which enters the citric acid cycle. Citrate synthase catalyzes this reaction.
Q.32Medium
Which of the following conditions causes increased glycogenolysis in muscle tissue during exercise?
Answer: B
During exercise, epinephrine increases and ATP/AMP ratio decreases, both of which activate phosphorylase kinase and promote glycogenolysis.
Q.33Medium
What is the primary fate of pyruvate under anaerobic conditions in muscle during intense exercise?
Answer: C
Under anaerobic conditions, pyruvate is reduced to lactate by lactate dehydrogenase (LDH) to regenerate NAD+ for continued glycolysis.
Q.34Medium
Which enzyme deficiency causes Pompe disease (GSD Type II)?
Answer: B
Pompe disease results from deficiency of acid α-glucosidase (lysosomal α-1,4-glucosidase), leading to accumulation of glycogen in lysosomes, especially in muscles and heart.
Q.35Medium
During the pentose phosphate pathway, which coenzyme is reduced to generate NADPH?
Answer: C
NADP+ is reduced to NADPH in the oxidative phase of the pentose phosphate pathway, primarily in the G6PD and 6-PGD reactions.
Q.36Medium
Which of the following best explains why glycogen is more soluble than starch despite similar glycosidic linkages?
Answer: A
Glycogen has more frequent α-1,6 branch points (approximately every 8-12 glucose units) compared to starch, increasing its solubility and accessibility for enzyme action.
Q.37Medium
In a patient with von Gierke disease, excessive hepatic glucose production is primarily due to:
Answer: C
Von Gierke disease (GSD Type I) involves glucose-6-phosphatase deficiency, leading to impaired glucose release. However, alternative glucose production through gluconeogenesis and pentose phosphate pathway increases.
Q.38Medium
Which of the following substrates can be directly utilized for glycogen synthesis in the liver?
Answer: B
Glucose-1-phosphate is converted to UDP-glucose by UDP-glucose pyrophosphorylase, which is the activated form used by glycogen synthase for glycogen synthesis.
Q.39Medium
A patient presents with inability to metabolize galactose. Which enzyme deficiency is most likely responsible for classical galactosemia?
Answer: B
Classical galactosemia results from galactose-1-phosphate uridyltransferase (GALT) deficiency, causing accumulation of galactose-1-phosphate and galactosylated proteins.
Q.40Medium
The branching enzyme in glycogen synthesis catalyzes which type of reaction?
Answer: B
Branching enzyme (amylo-1,6-transglucosidase) transfers segments of α-1,4-linked glucose chains to create α-1,6 branch points in glycogen.
