Step 1: Split CSOM into two patterns.
One pattern, tubotympanic disease, is a safe type with a central hole in the eardrum. The other, atticoantral disease, is unsafe and almost always carries cholesteatoma, skin growing where it should not, eating into bone.
Step 2: Ask what cholesteatoma needs.
Cholesteatoma keeps enlarging and eroding whatever bone is nearby, the ossicles, the facial nerve canal, even the inner ear. No drop or tablet dissolves this skin mass.
Step 3: Match the disease to the fix.
Since the problem is a physical, expanding mass, the fix has to be physical too, opening the mastoid and clearing every trace of the disease. That operation is mastoidectomy.
Step 4: Cross off the rest.
Medical treatment only quiets infection for a while. Underlay myringoplasty fixes a plain central perforation, which belongs to the safe type, not this one. A ventilation tube treats fluid behind an intact eardrum, a different problem altogether.
Step 5: Conclude.
$\text{Mastoidectomy}$ clears the cholesteatoma and stops the bone destruction.
\[ \boxed{\text{Mastoidectomy}} \]