This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Base substitution mutations can have the following molecular consequence except 
Select and explain. 
A. Changes one codon for an amino acid into another codon for that same amino acid, B. Codon for one amino acid is changed into a codon of another amino acid, C. Reading frame changes downstream to the mutant site, D. Codon for one amino acid is changed into a translation termination codon
C: Reading frame changes downstream to the mutant site
Answer: C: Reading frame changes downstream to the mutant site 
Explanation: Option 'a' is silent mutation, option 'b' is missense mutation, and option 'd' is nonsense mutation. All three are types of point mutation (base substitution mutation). Option 'c' is frame-shift mutation.