The overall appearance and large size of this nematode are strongly suggestive of A. lumbricoides, given that this is a human stool specimen. As Sam mentioned, examining the anterior end for the characteristic 3 fleshy lips common to all ascarids, and examination of the stool for eggs would allow for confirmation of the identification.
Here are some images from other cases to show these characteristic features:
Adult with 3 fleshy lips from Case of the Week 479:
Mammillated and decorticated eggs from Case of the Week 550
Thanks to all that wrote in with the answer, and to Dr. Poloni for donating this case!
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