Which of the following OTC drugs is commonly used to manufacture the street drug crystal methamphetamine?

Pseudoephedrine is the correct answer because it is a common precursor in the illicit manufacturing of methamphetamine, including crystal meth. In the legal pharmaceutical context, pseudoephedrine is found in many over-the-counter cold and allergy medications due to its properties as a decongestant. However, because of its potential for misuse in the synthesis of methamphetamine, regulations have tightened around its sale, and it is often subject to restrictions to monitor its purchase.

The other options listed do not have the same relationship to the production of illegal drugs. Diphenhydramine is an antihistamine primarily used for allergy relief and does not contribute to the synthesis of methamphetamine. Acetaminophen is a common pain reliever and fever reducer that has no connection to meth production. Ibuprofen is a nonsteroidal anti-inflammatory drug also used for pain and inflammation but is not involved in the creation of street drugs. Thus, while all these medications serve valid medicinal purposes, only pseudoephedrine is relevant in the context of methamphetamine manufacturing.