Q.1 – A 12-year-old boy develops convulsions. After an Electroencephalogram (EEG), a neurologist determines that the child has epilepsy. He is started on benzodiazepine, which promotes the activity of GABA. GABA is derived from Glutamate by which of the following reactions?
A) Deamination
B) Decarboxylation
C) Dehydrogenation
D) Hydroxylation
E) Transamination
Answer: B) Decarboxylation
Explanation: GABA (gamma-aminobutyric acid) is synthesized from glutamate primarily through a decarboxylation reaction, which removes a carboxyl group. This reaction is catalyzed by the enzyme glutamic acid decarboxylase.
Q.2 – A couple of African-American descent gives birth to a boy after an otherwise uneventful pregnancy. The child is exceptionally fair-skinned and has almost white hair. Further examination reveals red pupils. A postnatal screen is likely to confirm the deficiency of which of the following enzymes?
A) Cystathionine beta-synthase
B) Glutathione peroxidase
C) Glutathione reductase
D) Methionine synthase
E) Tyrosinase
Answer: E) Tyrosinase
Explanation: Tyrosinase is an enzyme essential for the synthesis of melanin, the pigment responsible for hair, skin, and eye color. A deficiency in tyrosinase results in albinism, characterized by light skin, white hair, and red pupils.
Q.3 – A 16-year-old girl is found by her parents unconscious on the bathroom floor with an empty bottle of acetaminophen in the toilet. She is rushed to the hospital, where she is given several doses of N-acetyl cysteine. Acetaminophen overdose is potentially life-threatening because it depletes the cellular stores of Glutathione. Which of the following amino acids is another component of Glutathione besides Cysteine and Glutamic acid?
A) Arginine
B) Glycine
C) Lysine
D) Methionine
E) Ornithine
Answer: B) Glycine
Explanation: Glutathione, a critical antioxidant in cells, is composed of three amino acids: glutamic acid, cysteine, and glycine. Glycine is essential for the molecule’s structure and function.
Q.4 – A 56-year-old man with long-standing poorly controlled diabetes mellitus visits his primary care physician for a follow-up after a recent hospitalization. The patient experienced an episode of acute renal failure while in the hospital, and his serum creatinine rose to 2.4 mg/dl (Normal, 0.7- 1.5 mg/dl). Creatinine, a marker of kidney function, is derived from which of the following precursors?
A) Arginine, Lysine, and Glutamine
B) Cysteine, Glycine, and Arginine
C) Glutamic acid, Cysteine, and Glycine
D) Methionine, Arginine, and Glycine
E) Methionine, Serine, and Glycine
Answer: D) Methionine, Arginine, and Glycine
Explanation: Creatinine is derived from creatine, which is synthesized from the amino acids arginine, glycine, and methionine. This process primarily occurs in the liver and is transported to muscles, where it is used or converted to creatinine.
Q.5 – S-Adenosyl Methionine (Active Methionine) is required to synthesize which of the following compounds?
A) Bile salts
B) Epinephrine
C) Melanin
D) Serotonin
E) Thyroid hormone
Answer: B) Epinephrine
Explanation: S-adenosyl methionine (SAMe) is a major methyl donor in numerous biochemical reactions, including the synthesis of epinephrine, which is a catecholamine.
Q.6 – Which of the following compounds is formed from hydroxylation requiring vitamin C and subsequent methylation?
A) Creatine
B) Dopamine
C) Epinephrine
D) Histamine
E) Melanin
Answer: C) Epinephrine
Explanation: Epinephrine synthesis involves the hydroxylation of tyrosine (requiring vitamin C) to L-DOPA, which is then converted to dopamine and subsequently to norepinephrine and epinephrine through methylation.
Q.7 – A 40-year-old woman complains of decreased energy, significant weight gain, and cold intolerance. She is seen by her family physician, who has diagnosed her to be having hypothyroidism (Low level of thyroid hormone). Which of the following is a precursor for thyroid hormone?
A) DOPA
B) Glutamine
C) Tyrosine
D) Tryptophan
E) Threonine
Answer: C) Tyrosine
Explanation: Tyrosine is a critical amino acid precursor for the synthesis of thyroid hormones. These hormones are produced by the iodination and coupling of tyrosine residues in the thyroid gland.
Q.8 – A 63-year-old woman reports a long history of joint pain. Her fingers are severely deformed, secondary to rheumatoid arthritis. Upon visiting a rheumatologist, she is started on methotrexate. This drug inhibits which of the following conversions?
A) Dopamine to norepinephrine conversion
B) Dihydrofolate to Tetrahydrofolate
C) N-Acetyl serotonin to melatonin
D) Phenylalanine to Tyrosine
E) Tyrosine to Dopa
Answer: B) Dihydrofolate to Tetrahydrofolate
Explanation: Methotrexate is a folate analog that inhibits the enzyme dihydrofolate reductase, which is responsible for the conversion of dihydrofolate to tetrahydrofolate. This action results in decreased DNA synthesis and cellular replication, particularly in rapidly dividing cells such as those involved in the immune response in rheumatoid arthritis.
Q.9 – A 59-year-old woman develops a shuffling gait and a pin-rolling tremor. She is referred to a neurologist for evaluation. After a thorough workup, a diagnosis of Parkinson’s disease is made, and the patient is placed on a Monoamine oxidase inhibitor. Which drug, in this case, is given to decrease the degradation of which of the following?
A) Dopamine
B) Melatonin
C) Nicotinamide
D) Nitric oxide
E) Serotonin
Answer: A) Dopamine
Explanation: In Parkinson’s disease, the key symptom of motor impairment is linked to the depletion of dopamine in the brain. Monoamine oxidase inhibitors are used to decrease the degradation of dopamine, thus increasing its availability and alleviating symptoms of the disease.
Q.10 – During a medical rotation, a medical student volunteered for a respiratory physiology examination that determines basal metabolic rate and the respiratory quotient. She followed the protocol for a resting individual in the post-absorptive state. Which of the following amino acids would be found in the highest concentration in the serum?
A) Alanine and Glutamine
B) Arginine and Ornithine
C) Branched-chain amino acids
D) Glutamate and Aspartate
E) Hydrophobic amino acids
Answer: A) Alanine and Glutamine
Explanation: In the post-absorptive state, alanine and glutamine are typically found in higher concentrations in the serum. These amino acids are crucial for transporting nitrogen and carbon from muscle tissue to the liver.
Q.11 – A 27-year-old semi-professional tennis player seeks advice from a hospital-based nutritionist concerning his diet supplements. His coach had given him amino acid supplements consisting of phenylalanine and tyrosine. The rationale was that these precursors to several neurotransmitters would “help his brain focus” on his game. In reality, the excess amino acids are used for energy, with a poor and eclectic diet. Phenylalanine, upon metabolism, enters the TCA cycle as which of the following?
A) α-Ketoglutarate
B) Citrate
C) Fumarate
D) Oxaloacetate
E) Succinyl-CoA
Answer: C) Fumarate
Explanation: Phenylalanine is metabolized into fumarate and acetoacetate and can thus enter the TCA cycle through Fumarate, linking it to energy metabolism.
Q.12 – In a 39-year-old woman who had just given birth, chorionic villus sampling was performed, and a battery of genetic panels was assessed on the newborn. One marker indicated a defective Cystathionine β-Synthase. Which of the following compounds would you most likely expect to be elevated in the blood of the infant at birth if the mother was not treated properly?
A) Cysteine
B) Glutamine
C) Methionine
D) Threonine
E) Valine
Answer: C) Methionine
Explanation: A defect in Cystathionine β-Synthase leads to homocystinuria, which often results in elevated levels of methionine in the blood. This enzyme is critical for converting homocysteine to cystathionine, and its deficiency disrupts this pathway, causing methionine and homocysteine to accumulate.
Q.13 – A 55-year-old man suffers from cirrhosis of the liver. Toxins such as ammonia are not properly metabolized by the liver and can damage the brain. Which of the following compounds is expected to be in the highest concentration in the brain as a result of the detoxification of ammonia?
A) Alpha-ketoglutarate
B) Asparagine
C) GABA
D) Glutamate
E) Glutamine
Answer: E) Glutamine
Explanation: Glutamine plays a crucial role in the detoxification of ammonia in the brain. In astrocytes, glutamate captures ammonia to form glutamine, which is then transported to the liver, where it can be safely converted to urea. This process helps to reduce the neurotoxic effects of ammonia, especially in liver dysfunction such as cirrhosis.
Q.14 – Which of the following enzymes requires adenosine triphosphate (ATP) to mediate its reactions?
A) Arginase
B) Argininosuccinate lyase
C) Argininosuccinate synthetase
D) Glutaminase
E) Ornithine transcarbamoylase
Answer: C) Argininosuccinate synthetase
Explanation: Argininosuccinate synthetase catalyzes the ATP-dependent condensation of citrulline and aspartate to form argininosuccinate, a key step in the urea cycle. This enzyme uses ATP to activate the carboxyl group of citrulline, facilitating the addition of aspartate.
Q.15 – Which of the following amino acids is not converted to Acetyl-CoA upon metabolism?
A) Leucine
B) Lysine
C) Tryptophan
D) Tyrosine
E) Valine
Answer: E) Valine
Explanation: Valine is one of the branched-chain amino acids and is primarily metabolized to succinyl-CoA, which enters the TCA cycle. Unlike some other amino acids, it does not directly contribute to the pool of acetyl-CoA but instead participates in gluconeogenesis and energy production through different pathways.
