**Core Concept**
Measles is a highly contagious viral infection caused by the measles virus. It primarily affects the respiratory system and is known for its distinctive rash. The measles virus can lead to severe complications, especially in vulnerable populations, such as young children and immunocompromised individuals.

**Why the Correct Answer is Right**
The most common cause of death in measles is secondary bacterial pneumonia. This occurs when the measles virus damages the lungs' cilia and mucociliary clearance mechanism, making it easier for bacteria to invade and cause pneumonia. The measles virus also suppresses the immune system, making it harder for the body to fight off secondary infections. Bacterial pneumonia can be caused by pathogens such as Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while encephalitis is a potential complication of measles, it is not the most common cause of death.
**Option B:** This option is incorrect because gastroenteritis is not a direct complication of measles, although it may occur due to secondary viral or bacterial infections.
**Option C:** This option is incorrect because while measles can lead to severe malnutrition, it is not the most common cause of death directly attributed to measles.

**Clinical Pearl / High-Yield Fact**
Measles can lead to severe complications, especially in young children, due to the suppression of the immune system and damage to the respiratory tract. Secondary bacterial pneumonia is the leading cause of death in measles, highlighting the importance of prompt treatment and vaccination against measles.

**Correct Answer: D**