Module 8 Exam: Amino Acid Metabolism, Nitrogen Balance, Urea
Cycle, Vitamins & Cofactors 2026/2027 UPDATE
1. Which of the following conditions is characterized by a positive nitrogen
balance?
A. Uncontrolled diabetes mellitus
B. Prolonged starvation
C. Advanced stage cancer
D. Severe burn recovery
Answer: D
Rationale: Positive nitrogen balance occurs when nitrogen intake exceeds excretion,
typically during periods of growth, pregnancy, or tissue repair/recovery from injury.
2. Which vitamin-derived cofactor is essential for all transamination reactions?
A. Thiamine pyrophosphate (TPP)
B. Flavin adenine dinucleotide (FAD)
C. Pyridoxal phosphate (PLP)
D. Nicotinamide adenine dinucleotide (NAD+)
Answer: C
Rationale: Pyridoxal phosphate (PLP), derived from Vitamin B6, serves as the essential
prosthetic group for aminotransferases in transamination.
,3. In the urea cycle, which molecule reacts with carbamoyl phosphate to form
citrulline?
A. Arginine
B. Ornithine
C. Aspartate
D. Fumarate
Answer: B
Rationale: Ornithine transcarbamoylase catalyzes the reaction between carbamoyl
phosphate and ornithine to produce citrulline in the mitochondria.
4. Which amino acid is considered purely ketogenic?
A. Leucine
B. Alanine
C. Valine
D. Tyrosine
Answer: A
Rationale: Leucine and Lysine are the only two purely ketogenic amino acids, meaning
they are degraded directly into acetyl-CoA or acetoacetate.
5. Where in the cell does the rate-limiting step of the urea cycle occur?
A. Cytosol
B. Mitochondrial matrix
C. Endoplasmic reticulum
D. Lysosome
Answer: B
Rationale: The formation of carbamoyl phosphate by CPS1, the rate-limiting step of the
urea cycle, occurs within the mitochondrial matrix.
, 6. What is the primary carrier of nitrogen from peripheral tissues to the liver
under non-exercise conditions?
A. Alanine
B. Glutamate
C. Pyruvate
D. Glutamine
Answer: D
Rationale: Glutamine serves as the non-toxic carrier of ammonia in the blood from various
tissues to the liver.
7. The Glucose-Alanine cycle primarily facilitates nitrogen transport between
which two organs?
A. Muscle and Liver
B. Liver and Kidney
C. Brain and Muscle
D. Intestine and Liver
Answer: A
Rationale: The Glucose-Alanine cycle transports amino groups from muscle to liver in the
form of alanine while returning glucose to the muscle.
8. Which enzyme is the primary regulator of the urea cycle and is activated by N-
acetylglutamate?
A. Arginase
B. Carbamoyl phosphate synthetase I
C. Ornithine transcarbamoylase
D. Argininosuccinate synthetase
Answer: B
Rationale: Carbamoyl phosphate synthetase I (CPS1) is the regulatory enzyme that
requires N-acetylglutamate (NAG) as an allosteric activator.
Cycle, Vitamins & Cofactors 2026/2027 UPDATE
1. Which of the following conditions is characterized by a positive nitrogen
balance?
A. Uncontrolled diabetes mellitus
B. Prolonged starvation
C. Advanced stage cancer
D. Severe burn recovery
Answer: D
Rationale: Positive nitrogen balance occurs when nitrogen intake exceeds excretion,
typically during periods of growth, pregnancy, or tissue repair/recovery from injury.
2. Which vitamin-derived cofactor is essential for all transamination reactions?
A. Thiamine pyrophosphate (TPP)
B. Flavin adenine dinucleotide (FAD)
C. Pyridoxal phosphate (PLP)
D. Nicotinamide adenine dinucleotide (NAD+)
Answer: C
Rationale: Pyridoxal phosphate (PLP), derived from Vitamin B6, serves as the essential
prosthetic group for aminotransferases in transamination.
,3. In the urea cycle, which molecule reacts with carbamoyl phosphate to form
citrulline?
A. Arginine
B. Ornithine
C. Aspartate
D. Fumarate
Answer: B
Rationale: Ornithine transcarbamoylase catalyzes the reaction between carbamoyl
phosphate and ornithine to produce citrulline in the mitochondria.
4. Which amino acid is considered purely ketogenic?
A. Leucine
B. Alanine
C. Valine
D. Tyrosine
Answer: A
Rationale: Leucine and Lysine are the only two purely ketogenic amino acids, meaning
they are degraded directly into acetyl-CoA or acetoacetate.
5. Where in the cell does the rate-limiting step of the urea cycle occur?
A. Cytosol
B. Mitochondrial matrix
C. Endoplasmic reticulum
D. Lysosome
Answer: B
Rationale: The formation of carbamoyl phosphate by CPS1, the rate-limiting step of the
urea cycle, occurs within the mitochondrial matrix.
, 6. What is the primary carrier of nitrogen from peripheral tissues to the liver
under non-exercise conditions?
A. Alanine
B. Glutamate
C. Pyruvate
D. Glutamine
Answer: D
Rationale: Glutamine serves as the non-toxic carrier of ammonia in the blood from various
tissues to the liver.
7. The Glucose-Alanine cycle primarily facilitates nitrogen transport between
which two organs?
A. Muscle and Liver
B. Liver and Kidney
C. Brain and Muscle
D. Intestine and Liver
Answer: A
Rationale: The Glucose-Alanine cycle transports amino groups from muscle to liver in the
form of alanine while returning glucose to the muscle.
8. Which enzyme is the primary regulator of the urea cycle and is activated by N-
acetylglutamate?
A. Arginase
B. Carbamoyl phosphate synthetase I
C. Ornithine transcarbamoylase
D. Argininosuccinate synthetase
Answer: B
Rationale: Carbamoyl phosphate synthetase I (CPS1) is the regulatory enzyme that
requires N-acetylglutamate (NAG) as an allosteric activator.
