A 37-year-old factory worker develops increasing weakness in the legs; coworkers have noted episodes of transient confusion. The patient has bilateral foot drop and atrophy; mild wrist weakness is also present. Deep tendon reflexes are absent in the lower extremities. His CBC shows an anemia with hemoglobin of 9.6 g/dL; examination of the peripheral blood smear shows basophilic stippling. Which of the following is the most likely cause of this patient's symptoms?

Lead poisoning often causes a peripheral neuropathy with primary motor involvement. It can superficially resemble ALS, but upper motor neuron signs (such as hyperreflexia) are not seen in lead poisoning. In addition, the cognitive changes of lead encephalopathy are not seen in early ALS, in peripheral nerve injuries (e.g., carpal or tarsal tunnel syndromes), or in myasthenia. Alcoholism can cause peripheral neuropathy but would not cause this patient's prominent motor weakness or the basophilic stippling. The presence of any anemia in a patient with peripheral neuropathy should prompt the clinician to think of a single disease entity (such as heavy metal poisoning) that can cause both entities. Lead lines may be seen at the gingiva-tooth border. Laboratory testing focuses on protoporphyrin levels (free erythrocyte or zinc) and blood lead levels. Industries often associated with lead exposure include battery and ceramic manufacturing, the demolition of lead-painted houses and bridges, plumbing, soldering, and, occasionally, exposure to the combustion of leaded fuels.