At what distance does a specific x-ray tube deliver a radiation output of 4 mR/mAs?

The correct distance at which a specific x-ray tube delivers a radiation output of 4 mR/mAs is 40 inches. This relationship between distance and radiation output is based on the inverse square law, which states that the intensity of radiation is inversely proportional to the square of the distance from the source.

At a distance of 40 inches, the radiation output measurement reflects a standard reference point used in radiographic practices. Testing at this set distance allows for consistent results and comparisons in radiation exposure in clinical settings. Typically, x-ray machines are calibrated to specific output readings at certain distances, and 40 inches is a common distance for these calibrations in fluoroscopy and other x-ray modalities.

The options represent different common measurement distances used in X-ray imaging. While the correct answer is 40 inches, each of the other distances may be relevant in other contexts, but they do not correspond to the output specified in the question. Hence, the chosen distance aligns with the expected output, confirming its accuracy for training and operational purposes.