Final answer:
Gentamicin is the medication among the ones listed (Rifampin, Gentamicin, and Isoniazid) that is known for causing ringing in the ears or tinnitus, due to its ototoxic effects.
Step-by-step explanation:
Out of the medications listed - Rifampin, Gentamicin, and Isoniazid - which the patient received for HIV and tuberculosis, Gentamicin is known to cause ototoxicity, which can manifest as ringing in the ears, a condition known as tinnitus. Although all these drugs have their own respective side effect profiles, only Gentamicin has a significant association with causing auditory side effects.
Rifampin and Isoniazid are primarily associated with hepatotoxicity and neurotoxicity, respectively, rather than ototoxicity. Therefore, for an HIV patient experiencing ringing in the ears after starting these medications, Gentamicin is the most likely culprit.