Step 1: The question pairs two findings, and the trick is that nephropathy is shared by most heavy metals, so it cannot be the discriminator.
Step 2: The decisive clue is the muscle pain, which is characteristically associated with arsenic toxicity.
Step 3: Putting the two together, the metal that produces both myalgia and kidney damage is arsenic.
Step 4: Cadmium, mercury, and lead can damage the kidney too, but they do not fit the myalgia hint, so arsenic is the best answer.
\[\boxed{\text{Arsenic}}\]