In the non-oxidative phase of the pentose phosphate pathway, ribulose-5-phosphate is converted to ribose-5-phosphate by the enzyme ribulose-5-phosphate isomerase. Which tissue shows MAXIMUM activity of this pathway and why?
ARed blood cells - for maintaining reducing power via NADPH
BAdipose tissue - for fatty acid synthesis requiring NADPH
CBone marrow - for rapid nucleotide synthesis and cell division
DLiver - for glycogen storage and gluconeogenesis
Correct Answer:
C. Bone marrow - for rapid nucleotide synthesis and cell division
EXPLANATION
Bone marrow shows the highest pentose phosphate pathway activity because rapidly dividing cells require maximum ribose-5-phosphate for nucleotide (DNA/RNA) synthesis during cell division. While adipose tissue needs NADPH for lipogenesis and RBCs need it for antioxidant defense, bone marrow's continuous hematopoiesis demands the most nucleotide precursors. This is clinically relevant in leukemia and chemotherapy patients.
A 45-year-old patient with Type 2 diabetes mellitus shows elevated fasting blood glucose (180 mg/dL) but normal HbA1c levels initially. Which carbohydrate metabolism pathway is primarily impaired in this patient's liver?
AGlycogenesis and increased gluconeogenesis
BGlycogenolysis and normal glycolysis
CPentose phosphate pathway upregulation
DLactate to pyruvate conversion
Correct Answer:
A. Glycogenesis and increased gluconeogenesis
EXPLANATION
Type 2 diabetes shows hepatic insulin resistance leading to impaired glycogenesis (reduced glycogen synthesis) and uncontrolled gluconeogenesis (excessive glucose production). This causes elevated fasting glucose despite normal HbA1c if glycemic control improves later. The liver fails to suppress glucose production in response to insulin.
Which adaptation occurs in liver during prolonged fasting to maintain blood glucose?
AIncreased glycogenolysis only
BIncreased gluconeogenesis from lactate and amino acids
CIncreased lipogenesis
DIncreased glycogen synthesis
Correct Answer:
B. Increased gluconeogenesis from lactate and amino acids
EXPLANATION
After 8-12 hours of fasting, hepatic glycogen depletes. The liver then relies on gluconeogenesis from Cori cycle lactate and amino acids to maintain blood glucose.
A diabetic patient shows impaired glucose utilization despite high blood glucose. Which enzyme's activity is most likely reduced?
AGlucokinase in beta cells
BPhosphofructokinase in muscle
CGlycogen phosphorylase in liver
DHexokinase in tissues
Correct Answer:
A. Glucokinase in beta cells
EXPLANATION
In diabetes, impaired insulin secretion (due to reduced glucokinase in beta cells) leads to inadequate glucose sensing and utilization by tissues despite hyperglycemia.
In the pentose phosphate pathway, the oxidative phase generates NADPH. Which metabolic process primarily utilizes this NADPH in fed state?
AGlycolysis
BKetone body synthesis
CFatty acid and cholesterol synthesis
DOxidative phosphorylation
Correct Answer:
C. Fatty acid and cholesterol synthesis
EXPLANATION
NADPH from the oxidative pentose phosphate pathway is essential for reductive biosynthesis of fatty acids and cholesterol, which occur predominantly in the fed state.
A 6-year-old child presents with muscle pain and myoglobinuria after exercise. Muscle biopsy shows normal glycogen structure but elevated content. This is consistent with:
ABranching enzyme deficiency
BMuscle phosphorylase deficiency
CLysosomal acid maltase deficiency
DLiver phosphorylase deficiency
Correct Answer:
B. Muscle phosphorylase deficiency
EXPLANATION
McArdle disease (GSD Type V) results from muscle phosphorylase deficiency. Glycogen accumulates but is structurally normal. Exercise intolerance and myoglobinuria are characteristic.
