**Core Concept**
The maximum fraction of inspired oxygen (FiO₂) achievable through a nasal oxygen catheter is limited by the device's design and the patient's airway dead space. Nasal cannulas mix oxygen with ambient air, and the resulting FiO₂ depends on the flow rate and the proportion of oxygen in the inspired gas.

**Why the Correct Answer is Right**
In a nasal cannula, oxygen is delivered through two prongs that mix with room air. The maximum FiO₂ is determined by the **dead space** and **flow rate**. Theoretical calculations show that with a standard nasal cannula, the highest achievable FiO₂ is approximately **44%**, due to the mixing of oxygen with room air (which contains 21% O₂). This is derived from the formula: FiO₂ = (flow rate × 100) / (flow rate + 100), where flow is in liters per minute. For a typical flow rate of 6 L/min, FiO₂ ≈ 44%.

**Why Each Wrong Option is Incorrect**
Option A: A FiO₂ of 1 (100%) is impossible with nasal cannulae due to air mixing and dead space.
Option B: 0.6 (60%) is not achievable; it exceeds the physical limits of nasal cannula mixing.
Option C: 0.2 (20%) is the oxygen concentration in room air, not the maximum deliverable by nasal cannula.

**Clinical Pearl / High-Yield Fact**
Nasal cannula is best used for low to moderate oxygen needs; for higher FiO₂ (>60%), non-rebreather masks or high-flow oxygen systems are required. Always consider dead space and mixing when selecting oxygen delivery devices.

✓ Correct Answer: D. 0.44