**Core Concept**
Acute mastoiditis is an inflammatory condition of the mastoid bone, typically as a complication of acute otitis media. The condition arises due to the spread of infection from the middle ear to the mastoid air cells. Radiological features of acute mastoiditis are crucial for diagnosis and assessment of disease severity.

**Why the Correct Answer is Right**
The essential radiological feature of acute mastoiditis is clouding of the air cells of the mastoid. This occurs due to the accumulation of pus and inflammatory cells within the mastoid air cells, which appears as a cloudy or opacified appearance on radiographs. This is a direct result of the spread of infection from the middle ear to the mastoid air cells. The clouding of the air cells is often accompanied by destruction of the cortical bone and expansion of the mastoid air cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Temporal bone pneumatization is a normal anatomical feature, which can sometimes be overestimated in radiological assessment. However, it is not a feature of acute mastoiditis. The condition does not involve an increase in pneumatization but rather an inflammatory process.
**Option C:** Rarefaction of the petrous bone is not a characteristic feature of acute mastoiditis. Rarefaction refers to the decrease in bone density, which can occur in conditions like otosclerosis. In acute mastoiditis, the cortical bone is often destroyed, but this is not the same as rarefaction.
**Option D:** Thickening of the temporal bone is not a specific feature of acute mastoiditis. Thickening can occur in various conditions, including chronic otitis media and temporal bone fractures. In acute mastoiditis, the cortical bone is often destroyed, leading to expansion and clouding of the air cells.

**Clinical Pearl / High-Yield Fact**
In acute mastoiditis, the clouding of the air cells is often accompanied by a decrease in the mastoid tip distance, which can be a useful radiological indicator of disease severity.

**✓ Correct Answer: B. Clouding of air cells of mastoid**