**Core Concept**
Severe pneumonia is a life-threatening condition characterized by impaired gas exchange, hypoxemia, and respiratory failure. The World Health Organization (WHO) defines severe pneumonia based on clinical and radiological criteria, including chest X-ray findings, respiratory rate, and oxygen saturation.

**Why the Correct Answer is Right**
The most important feature to diagnose severe pneumonia is the presence of a new or worsening infiltrate on chest X-ray, particularly if it involves more than 50% of the lung field. This is often accompanied by signs of respiratory distress, such as tachypnea (increased respiratory rate), tachycardia (increased heart rate), and altered mental status. The infiltrate may be due to alveolar consolidation, which occurs when inflammatory cells and fluid accumulate in the alveoli, impairing gas exchange.

**Why Each Wrong Option is Incorrect**
**Option A:** Increased white blood cell count is a nonspecific marker of infection but does not specifically diagnose severe pneumonia.
**Option B:** Elevated body temperature is a common symptom of pneumonia but is not a reliable indicator of severity.
**Option C:** Hypotension is more commonly associated with septic shock, a condition that requires immediate recognition and treatment.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with suspected severe pneumonia, always obtain a chest X-ray to identify signs of consolidation, such as lobar infiltrates or cavitation.

**Correct Answer: C. Hypotension.**