**Core Concept**
Respiratory distress is a clinical syndrome characterized by an inability to breathe normally, often accompanied by feelings of suffocation, anxiety, or panic. This condition can arise from various etiologies, including pulmonary, cardiac, or neurological causes.

**Why the Correct Answer is Right**
Respiratory distress is typically caused by an imbalance between the demand for oxygen and the ability of the respiratory system to supply it. This can occur due to conditions such as pneumonia, acute respiratory distress syndrome (ARDS), or chronic obstructive pulmonary disease (COPD). In these cases, the lungs may become inflamed, damaged, or obstructed, leading to impaired gas exchange and subsequent respiratory distress.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not a recognized cause of respiratory distress.
* **Option B:** This option is not a relevant condition in the context of respiratory distress.
* **Option C:** This option is incorrect because while it may be a symptom of respiratory distress, it is not a direct cause.

**Clinical Pearl / High-Yield Fact**
It is essential to differentiate between respiratory distress and respiratory failure, as the latter is a more severe condition requiring immediate medical intervention.

**Correct Answer: D. Pneumonia**