Question

why is it challenging for doctors to treat certain brain - related conditions? because they need to find drugs that are not already in use for other conditions. because they need to find drugs that can be administered orally. because they need to find drugs that can be produced at a low cost. because they need to find drugs that can cross the blood - brain barrier. because they need to find drugs that can target specific neurons.

Explanation:

The blood - brain barrier is a highly selective semi - permeable border that prevents many substances from reaching the brain. Drugs need to cross this barrier to treat brain - related conditions, which is a major challenge in brain - condition treatment.

Answer:

Because they need to find drugs that can cross the blood - brain barrier.