In the context of patient complaints, what is the correct formulation?

The formulation "The patient complains of diarrhea" is correct because it uses the appropriate preposition "of" to indicate what the patient is expressing concern about. In English, the verb "complain" is typically followed by "of" when referring to a specific health issue or symptom that the subject is experiencing. This structure is standard in medical and clinical contexts, ensuring clear communication about the patient's condition.

The other options do not adhere to the commonly accepted grammatical rules for using "complain." For instance, "complains for diarrhea" introduces an incorrect preposition, while "complain of diarrhea" lacks subject-verb agreement due to the absence of an "s" at the end of "complain" to match the singular subject "patient." Lastly, "complains about diarrhea" can be used in informal contexts, but using "about" is less precise when discussing medical symptoms compared to the more formal "complains of." Thus, the correct answer maintains clarity and aligns with standard English usage in healthcare communication.