What happens from a mutation?
As with any catalyzing molecule in the metabolic processes, having a mutation highly reduces the changes that the organism will survive. Without the ability to break down foods, the cell can no longer function. However, depending when the mutation occurs can severely alter the ability to live.
If the mutation occurs in DNA replication at an early stage of infant development, the chances of survival are slim. Without stopping the initial MDH gene mutation all daughter cells will be influenced and will largely go unstopped. This would be fatal; regardless if MDH is null or inefficient will certainly have an effect on the development and growth of the fetus.
If the mutation occurs in transcription or translation, this gives the cell a chance to find a re-route, or another pathway that is useful to the cell. The cell might even undergo another mutation that negates the mutation of MDH. Either or, having a MDH gene mutation is likely terminal but there is a slight increase in survival if the mutation occurs in transcription or translation. Why? Because the cell will simply die and the body will create anew.
Mutations can occur for chemical or physical agents that alter the shape or even code of the DNA. Mutagens can include X-rays, chemical warfare agents, and UV rays.