**Core Concept**
Ventilation-perfusion (VA/Q) ratio represents the balance between airflow (ventilation) and blood flow (perfusion) in the lungs. A VA/Q ratio of infinity indicates no perfusion (blood flow) while ventilation is present—this is the definition of **dead space** ventilation.

**Why the Correct Answer is Right**
When VA/Q is infinity, it means ventilation occurs but there is no blood flow (Q = 0). This results in alveolar air not being oxygenated or carbon dioxide removed—hence, it is **anatomical dead space** or **physiological dead space**. No gas exchange occurs, so the alveoli are ventilated but not perfused. This is distinct from shunting or atelectasis.

**Why Each Wrong Option is Incorrect**
Option B: The stated PO2 and PCO2 values (159 mmHg O2, 40 mmHg CO2) reflect normal alveolar gas, not infinity VA/Q. Such values occur in well-perfused, well-ventilated areas, not in absent perfusion.
Option C: Equal partial pressures of O2 and CO2 are not a feature of infinity VA/Q; this would imply gas exchange equilibrium, which does not occur in dead space.
Option D: Atelectasis refers to collapsed alveoli, which causes **low VA/Q**, not infinity. In atelectasis, perfusion is reduced but not zero, so VA/Q is finite and low, not infinite.

**Clinical Pearl / High-Yield Fact**
VA/Q infinity = **no perfusion**, = **dead space**. Remember: **"Infinite VA/Q = ventilation without perfusion"** — this is the hallmark of **dead space**. This concept is critical in diagnosing pulmonary embolism (PE), where perfusion drops to zero in some areas.

✓ Correct Answer: A. Dead space