A patient with galactosemia has a deficiency in which enzyme?
Answer: B
Classic galactosemia results from deficiency of galactose-1-phosphate uridyltransferase (GALT), preventing conversion of galactose-1-phosphate to UDP-galactose. Leads to accumulation of galactose-1-phosphate and galactitol (in polyol pathway), causing cataracts, intellectual disability, and liver damage.
Q.2Hard
Pasteur effect refers to the inhibition of glycolysis by oxidative phosphorylation. In terms of ATP and citrate, which statement is correct?
Answer: D
PFK is inhibited by high ATP, citrate (signal of sufficient acetyl-CoA), and low pH. In aerobic conditions, efficient oxidative phosphorylation produces sufficient ATP, reducing need for glycolysis (Pasteur effect). Citrate is an allosteric inhibitor of PFK.
Q.3Hard
In glycolysis, which step is irreversible under physiological conditions and requires a different enzyme during gluconeogenesis?
Answer: B
The PFK reaction is highly exergonic (ΔG°' = -14.2 kJ/mol) and is essentially irreversible. During gluconeogenesis, fructose-1,6-bisphosphatase catalyzes the reverse reaction. This is a major control point in carbohydrate metabolism.
Q.4Hard
The Warburg effect describes increased glycolysis in cancer cells even in the presence of oxygen. Which enzyme is typically upregulated in cancer cells to support this?
Answer: C
In the Warburg effect, PFK-2 and PKM2 are upregulated. PFK-2 produces fructose-2,6-bisphosphate (a potent PFK-1 activator), while PKM2 (pyruvate kinase isoform) is upregulated in cancer cells. PKM2 also has non-glycolytic functions in cancer metabolism.
Q.5Hard
A 6-month-old infant develops hypoglycemia, hepatomegaly, and lactic acidosis after feeding. Genetic testing reveals glucose-6-phosphatase deficiency (Type I Glycogen Storage Disease). Why does this cause lactic acidosis?
Answer: A
G6Pase is the final enzyme in both gluconeogenesis and glycogenolysis. Its deficiency traps glucose-6-phosphate, forcing it through glycolysis and the pentose phosphate pathway, increasing pyruvate and lactate production.
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Q.6Hard
A competitive athlete is found to have a deficiency in muscle phosphorylase (McArdle disease). During intense exercise, which metabolic consequence is PRIMARY?
Answer: A
Muscle phosphorylase deficiency prevents glycogen breakdown, depriving muscles of glucose-1-phosphate during exercise, causing severe energy crisis, fatigue, cramps, and myoglobinuria.
Q.7Hard
A patient with hemoglobin C disease (defect in β-globin) shows increased levels of fetal hemoglobin (HbF). Why might elevated HbF reduce hemolysis compared to HbS?
Answer: B
While this is primarily a hemoglobin question, HbF (with γ-chains instead of β-chains) does not polymerize like HbS or aggregate like HbC, reducing hemolysis and RBC sickling/crystallization.
Q.8Hard
A 3-year-old child presents with hepatomegaly, growth retardation, and elevated liver transaminases. Enzyme assay shows deficiency of lysosomal acid glucosidase (Pompe disease/GSD Type II). Which carbohydrate accumulates PRIMARILY in lysosomes?
Answer: C
In Pompe disease, acid α-glucosidase deficiency prevents lysosomal glycogen hydrolysis. Normally structured glycogen accumulates in lysosomes (unlike the abnormal structures seen in Type IV GSD), causing lysosomal dysfunction and cellular damage.
Q.9Hard
Which of the following correctly pairs a glycogen storage disease with its enzyme defect and primary organ affected?
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.11Hard
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.12Hard
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.13Hard
A 45-year-old male presents with chronic hyperuricemia and gout. Testing reveals elevated lactic acid and hepatomegaly. Which GSD is most likely?
Answer: B
Von Gierke disease causes hepatomegaly, lactic acidosis, and hyperuricemia due to glucose-6-phosphatase deficiency, leading to increased glycolysis and purine metabolism.
Q.14Hard
In the Cori cycle, lactate produced in muscle is converted to glucose in the liver. Which enzyme is crucial for the final step in hepatic gluconeogenesis?
Answer: B
Glucose-6-phosphatase catalyzes the dephosphorylation of glucose-6-phosphate to free glucose, which is the final and rate-limiting step of hepatic gluconeogenesis.
Q.15Hard
Which adaptation occurs in skeletal muscle during prolonged fasting (>48 hours)?
Answer: B
After 24-48 hours of fasting, muscle glycogen is depleted. Muscle shifts to oxidizing amino acids (from proteolysis) and utilizing ketone bodies produced by the liver.
Q.16Hard
A patient with G6PD deficiency may develop hemolytic anemia upon exposure to oxidative stress. This occurs because:
Answer: A
G6PD deficiency reduces NADPH production, decreasing reduced glutathione (GSH) levels. GSH protects RBC membranes from oxidative damage, so its depletion leads to hemolysis.
Q.17Hard
Which of the following correctly describes the allosteric regulation of glycogen phosphorylase in muscle?
Answer: C
In muscle, phosphorylase a (phosphorylated form) is active. It is further activated by AMP, which signals energy depletion during exercise.
Q.18Hard
In maple syrup urine disease (MSUD), accumulation of branched-chain amino acids affects carbohydrate metabolism by:
Answer: C
BCAA metabolism produces acetyl-CoA and increases the acetyl-CoA/CoA ratio, which inhibits pyruvate dehydrogenase, reducing glucose oxidation and affecting carbohydrate metabolism.
Q.19Hard
Which statement accurately describes the relationship between blood glucose regulation and the Warburg effect in cancer cells?
Answer: D
The Warburg effect describes the metabolic shift in cancer cells toward anaerobic glycolysis, producing lactate even in the presence of oxygen, resulting in high glucose consumption.
Q.20Hard
Which statement best explains the structural basis for the higher energy content of glucose compared to other hexoses?
Answer: C
All hexoses yield approximately the same amount of ATP (~32-38 ATP) through complete oxidation. The ATP yield depends on the metabolic pathways utilized, not intrinsic energy differences.
