What could a unilateral ear discharge suggest?

Unilateral ear discharge can be indicative of conditions affecting the ear, particularly otitis externa or a perforated tympanic membrane. Otitis externa, also known as swimmer's ear, is an infection of the outer ear canal, which can cause discharge that is typically clear, yellow, or green and often accompanied by pain, itching, and inflammation. When the tympanic membrane is perforated, it can also result in discharge, which may contain pus or blood and be associated with ear pain, hearing loss, and sometimes fever.

In contrast, the other options provided do not typically lead to unilateral ear discharge. Sinus infections can cause nasal discharge or postnasal drip but are not directly associated with ear discharge. Allergic reactions may lead to congestion or itching but usually do not cause purulent discharge from the ear. Lastly, a common cold primarily affects the upper respiratory tract and is characterized by symptoms such as a runny nose and sore throat, rather than discharge from the ear. Therefore, the connection between unilateral ear discharge and either otitis externa or a perforated tympanic membrane is strong, making this the correct choice.