**Core Concept**
Chronic Suppurative Otitis Media (CSOM) is a long-standing infection of the middle ear that can lead to various complications due to its chronic nature and the potential for bacterial invasion into adjacent structures. The common complications of CSOM are often related to the spread of infection to the surrounding tissues and the development of secondary conditions such as hearing loss.

**Why the Correct Answer is Right**
The commonest complication of CSOM is meningitis, which occurs when the infection spreads to the meninges, the protective membranes covering the brain and spinal cord. This happens when the infection from the middle ear spreads through the petrosal sinus or the Eustachian tube, leading to inflammation of the meninges and potentially life-threatening consequences. The petrosal sinus plays a crucial role in this process, as it is a venous channel that connects the middle ear to the cavernous sinus, which in turn drains into the superior sagittal sinus, potentially leading to the spread of infection to the brain.

**Why Each Wrong Option is Incorrect**
**Option A:** Malignant otitis externa is a severe infection of the external auditory canal, but it is not the commonest complication of CSOM.
**Option B:** Cholesteatoma is a type of skin cyst that can develop in the middle ear due to CSOM, but it is not the most common complication.
**Option C:** Sensorineural hearing loss can occur due to CSOM, but it is not the most common complication.

**Clinical Pearl / High-Yield Fact**
CSOM can lead to serious complications if not managed promptly, and the spread of infection to the meninges is a potential consequence. Early diagnosis and treatment are crucial in preventing these complications.

**Correct Answer: C. Meningitis. Meningitis.**