**Core Concept**
Pertussis, also known as whooping cough, is a highly contagious respiratory illness caused by the bacterium Bordetella pertussis. It primarily affects infants and young children, leading to severe coughing fits that can be life-threatening.

**Why the Correct Answer is Right**
Cerebellar ataxia is not a known feature of pertussis. The disease primarily affects the respiratory system, causing severe coughing fits, and can lead to complications such as pneumonia, bronchiectasis, and encephalopathy due to hypoxia and increased intracranial pressure. However, cerebellar ataxia is not a characteristic manifestation of pertussis. In contrast, subconjunctival hemorrhage can occur due to intense coughing, and bronchiectasis is a potential long-term complication of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Encephalopathy is a potential complication of pertussis, particularly in young infants, due to hypoxia and increased intracranial pressure. Therefore, it is a feature of pertussis.
**Option C:** Subconjunctival hemorrhage can occur in pertussis due to intense coughing, making it a feature of the disease.
**Option D:** Bronchiectasis is a potential long-term complication of pertussis, leading to chronic respiratory problems in affected individuals.

**Clinical Pearl / High-Yield Fact**
Pertussis is a highly contagious disease that can be prevented through vaccination. The vaccine is typically administered in a series of doses during childhood, and booster shots are recommended to maintain immunity throughout life.

**✓ Correct Answer: B. Cerebellar Ataxia**