Double replacement reaction, also called double displacement reaction is a type of reaction in which the two reactants exchange there cations or anions as represented below
AB + CD ⇒ AD + CB
The reaction provided in the question is expected to produce the products shown below
K₂SO₄ + Pb(NO₃)₂ ⇒ PbSO₄ + 2KNO₃
As you can see from the equation above, both potassium (K) in K₂SO₄ and lead (Pb) in Pb(NO₃)₂ exchanged there positions hence the reaction is a double displacement which formed PbSO₄ and KNO₃
When a patient developing a resistance to a certain drug, the same amount of doses will not exert the same effect as before. The effect will be lower, thus the doctor will need to increase the dose to increase the effect