Final answer:
A patient with pain in the lower back and calves when walking, which is relieved by rest, is likely suffering from peripheral artery disease (PAD). An Ankle-Brachial Index (ABI) measurement is the test most likely to help diagnose PAD by detecting reduced blood flow in the legs.
Step-by-step explanation:
The patient's complaint of pain when walking that radiates from the lower back to the calves and is relieved by rest suggests a diagnosis of peripheral artery disease (PAD). PAD is characterized by the narrowing of peripheral arteries due to atherosclerosis, commonly affecting the legs, and can cause intermittent claudication, which is pain during exertion that subsides with rest. To yield a diagnosis, the test most likely to be informative would be an Ankle-Brachial Index (ABI) measurement, which compares the blood pressure in the ankle with the blood pressure in the arm to detect PAD. Additionally, other noninvasive tests such as Doppler ultrasound and angiography can be used to visualize the blood flow and identify any blockages in the arteries.