MBBS Biochemistry 3

Question 1

Which amino acid is the precursor for catecholamine synthesis (dopamine, norepinephrine, epinephrine)?

Accepted Answer

Phenylalanine/Tyrosine

Answer

Tyrosine (derived from phenylalanine) is converted to L-DOPA and then sequentially to dopamine, norepinephrine, and epinephrine in catecholamine synthesis.

Question 2

Cystic fibrosis is caused by a defect in which protein?

Accepted Answer

CFTR (Cystic Fibrosis Transmembrane conductance Regulator)

Answer

CF is caused by mutations in the CFTR gene encoding a chloride channel; the most common mutation (ΔF508) causes protein misfolding and failure to reach the cell surface.

Question 3

Which reaction produces the most ATP per molecule of substrate in cellular respiration?

Accepted Answer

Oxidative phosphorylation via the electron transport chain

Answer

Oxidative phosphorylation via the electron transport chain generates approximately 32–34 ATP per glucose, far exceeding the 2 ATP from glycolysis and 2 GTP from the Krebs cycle.

Question 4

Which cofactor is required for carboxylation reactions in fatty acid synthesis and gluconeogenesis?

Accepted Answer

Biotin

Answer

Biotin is covalently bound to carboxylases (pyruvate carboxylase, acetyl-CoA carboxylase) and acts as a CO2 carrier in carboxylation reactions.

Question 5

In the urea cycle, the final step converts arginine to urea and which other product?

Accepted Answer

Ornithine

Answer

Arginase cleaves arginine to yield urea (excreted) and ornithine, which re-enters the cycle to accept another carbamoyl group.

MBBS - Bachelor of Medicine and Bachelor of Surgery Practice Test

MBBS - Bachelor of Medicine and Bachelor of Surgery Practice Test

MBBS Biochemistry 3