The Cori cycle operates between muscle and liver. Which of the following correctly describes the fate of lactate in the liver?
Answer: B
The Cori cycle involves lactate produced in muscles being transported to the liver where it undergoes gluconeogenesis to form glucose, which is sent back to muscles. This is crucial during anaerobic exercise.
Q.142Easy
Which of the following is a ketohexose that is a structural isomer of glucose?
Answer: A
Fructose is a ketohexose (6-carbon ketone sugar) with the molecular formula C₆H₁₂O₆, making it a structural isomer of glucose. Galactose and mannose are aldohexoses (epimers of glucose), while ribose is a pentose.
Q.143Medium
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.144Easy
Which disaccharide is formed by α-1,4-glycosidic linkage between glucose molecules?
Answer: C
Maltose consists of two glucose units linked by an α-1,4-glycosidic bond. Sucrose contains glucose and fructose, lactose contains glucose and galactose, and trehalose has an α-1,1-glycosidic linkage.
Q.145Medium
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.
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Q.146Medium
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.147Medium
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.148Medium
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.
Q.149Easy
Which carbohydrate is a non-reducing sugar that cannot undergo mutarotation?
Answer: B
Sucrose is a non-reducing disaccharide formed by α-1,2-glycosidic linkage between C1 of glucose and C2 of fructose, blocking both anomeric carbons. It cannot mutarotate or act as a reducing sugar. Other options are reducing sugars with free anomeric carbons.
Q.150Medium
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.151Medium
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.152Medium
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.153Hard
Which of the following correctly pairs a glycogen storage disease with its enzyme defect and primary organ affected?
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.155Hard
Which monosaccharide cannot be directly metabolized by red blood cells due to lack of specific enzymes?
Answer: C
RBCs lack galactokinase and UDP-galactose-4-epimerase, making them unable to utilize galactose. They can metabolize glucose, fructose (via hexokinase), and mannose. This is relevant to understanding galactosemia pathophysiology where galactose accumulates in RBCs.
Q.156Hard
In the Pasteur effect, the inhibition of glycolysis by oxidative phosphorylation is primarily mediated by which molecule(s)?
Answer: D
The Pasteur effect describes how aerobic respiration inhibits glycolysis through multiple mechanisms: increased ATP/AMP ratio (inhibiting PFK-1), increased NADH/NAD⁺ ratio (inhibiting GAPDH), and increased citrate (allosteric inhibitor of PFK-1). This explains why cells prefer oxidative metabolism when oxygen is available.
Q.157Hard
A newborn presents with jaundice, hepatomegaly, and infantile cataracts. Laboratory findings show elevated galactose in blood and urine. Which enzyme deficiency is most likely?
Answer: C
Classical galactosemia results from galactose-1-phosphate uridylyltransferase (GALT) deficiency, causing accumulation of galactose-1-phosphate which is toxic to liver, brain, and lens. This leads to the classic triad of neonatal jaundice, hepatomegaly, and cataracts. Early dietary restriction of lactose prevents complications.
Q.158Medium
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.159Easy
Which of the following carbohydrates is classified as a pentose sugar?
Answer: A
Ribose is a 5-carbon (pentose) sugar, while glucose, galactose, and fructose are hexoses (6-carbon sugars).
Q.160Easy
In glycolysis, the conversion of glucose-6-phosphate to fructose-6-phosphate is catalyzed by which enzyme?
Answer: B
Phosphoglucose isomerase catalyzes the isomerization of glucose-6-phosphate to fructose-6-phosphate in the second step of glycolysis.
