Final answer:
The client taking ethambutol (Myambutol) should discontinue the medication if they experience visual disturbances, which are indicative of ethambutol's known side effect of optic neuritis.
Step-by-step explanation:
The manifestation reported by the client that would necessitate discontinuation of ethambutol is b) Visual disturbances. Ethambutol is known to cause optic neuritis, which can lead to visual disturbances such as loss of visual acuity and color blindness. This side effect is particularly concerning and warrants the discontinuation of the drug to prevent possible permanent damage to the patient's vision. On the other hand, peripheral neuropathy is more commonly associated with isoniazid (INH), hepatotoxicity can occur with several anti-TB drugs including INH, and ototoxicity is not typically associated with ethambutol but rather with drugs like aminoglycosides.