D

Step 1: Understand the conditions of the process. An isothermal process is one that occurs at a constant temperature. For an ideal gas, the equation of state is given by the Ideal Gas Law:
$$PV = nRT$$
where P is pressure, V is volume, n is the number of moles, R is the ideal gas constant, and T is the absolute temperature.

Step 2: In the isothermal process, the product $PV$ remains constant since both n and R remain constant for a given amount of gas at a specific temperature.

Step 3: The equation given in the question is $PV^\gamma = constant$. However, for an isothermal process, this does not hold true as \gamma (the heat capacity ratio) comes into play in adiabatic processes, not isothermal ones.
Therefore, for the isothermal case, we assert:
$$PV = constant$$ which is not the same as $$PV^\gamma = constant$$.

Conclusion: Since PV^γ = constant does not describe an isothermal expansion, the correct answer is option D.