**Core Concept**
Post-measles death is often due to secondary bacterial infections or complications arising from the measles virus itself. Measles is a highly contagious viral disease characterized by fever, cough, runny nose, and a characteristic rash. The measles virus targets the respiratory system and the immune system, making individuals more susceptible to secondary infections.

**Why the Correct Answer is Right**
The most common cause of post-measles death is pneumonia, particularly bacterial pneumonia caused by Streptococcus pneumoniae, Haemophilus influenzae, or Staphylococcus aureus. These opportunistic pathogens take advantage of the compromised respiratory system and the weakened immune response caused by the measles virus. The measles virus itself also causes damage to the epithelial lining of the respiratory tract, making it easier for bacteria to colonize and cause infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** While encephalitis is a serious complication of measles, it is not the most common cause of post-measles death. Encephalitis is a rare but potentially life-threatening complication, but it is not as prevalent as pneumonia.
* **Option B:** Diarrhea and dehydration are common complications of measles, but they are not the most common cause of post-measles death. While severe diarrhea can lead to dehydration and electrolyte imbalances, it is not typically fatal.
* **Option D:** Bronchitis is a common complication of measles, but it is not the most common cause of post-measles death. While bronchitis can be severe and debilitating, it is not typically fatal.

**Clinical Pearl / High-Yield Fact**
Measles is a leading cause of pneumonia and mortality in young children worldwide. The measles vaccine is highly effective in preventing measles and its complications, making it a crucial component of childhood vaccination schedules.

**Correct Answer:** C. Pneumonia.