**Core Concept**
The best view for detecting a hollow viscus perforation is crucial in radiological diagnosis. A hollow viscus, or gastrointestinal tract, perforation can lead to free air under the diaphragm, visible on an upright chest X-ray. This is due to the difference in air density between the lung and the free air.

**Why the Correct Answer is Right**
The erect chest X-ray is the best view for detecting a hollow viscus perforation because it allows free air to rise to the highest point in the thoracic cavity, making it more visible under the diaphragm. This is due to the principle of radiolucency, where air appears darker on a radiograph. In an erect position, free air can accumulate under the diaphragm, creating a characteristic "cupola" sign or "air under the diaphragm" sign. This sign is indicative of a perforated hollow viscus.

**Why Each Wrong Option is Incorrect**
**Option B:** A supine chest X-ray is not ideal for detecting free air under the diaphragm because the air will be trapped in the dependent portions of the thoracic cavity.
**Option C:** A right lateral chest X-ray may not be as sensitive as an erect view for detecting free air under the diaphragm, as the air may not rise to the highest point in the thoracic cavity in this position.
**Option D:** A left lateral chest X-ray is also not ideal for detecting free air under the diaphragm for the same reason as option C.

**Clinical Pearl / High-Yield Fact**
Remember the "3 Ps" of free air under the diaphragm: **position** (erect view), **presence** (air under the diaphragm), and **pattern** (cupola sign).

**✓ Correct Answer: A. Erect**