Cerumen Impaction: This is the most common cause of symptoms related to earwax, where the earwax blocks the ear canal, leading to hearing loss, discomfort, or a feeling of fullness in the ear. The justification for this being the single most likely diagnosis is the high prevalence of cerumen impaction in the general population and its direct association with the symptoms described.

Outer Ear Infection (Otitis Externa): An infection of the outer ear canal, which can cause symptoms similar to impacted earwax, including itching, redness, and discharge. This diagnosis is likely because the symptoms can overlap, and otitis externa is a common condition.
Middle Ear Effusion: Fluid accumulation in the middle ear, which can cause hearing loss and a feeling of fullness, similar to impacted earwax. This condition is a likely differential due to its potential to mimic the symptoms of earwax impaction.
Eustachian Tube Dysfunction: A condition where the tube that connects the middle ear to the back of the nose and throat doesn't open and close properly, leading to a feeling of fullness or pressure in the ear. This is considered because its symptoms can be confused with those of impacted earwax.

Acoustic Neuroma: A rare, benign tumor on the nerve connecting the inner ear to the brain, which can cause hearing loss, tinnitus, and balance problems. Although rare, missing this diagnosis could have significant implications for the patient's health and quality of life.
Mastoiditis: An infection of the mastoid bone in the skull, located behind the ear, which can cause symptoms similar to otitis externa or middle ear infections but requires prompt treatment to prevent serious complications. This is a "do not miss" diagnosis due to its potential severity.
Temporal Bone Fracture: A fracture of the bone in the skull that contains the structures of the ear, which can cause hearing loss, vertigo, and facial paralysis. The potential severity of this condition necessitates its inclusion as a "do not miss" diagnosis.

Cholesteatoma: A type of skin cyst located in the middle ear, which can cause destruction of the surrounding bone and tissue, leading to hearing loss, vertigo, and other neurological symptoms. This condition is rare but can have serious consequences if not treated.
Relapsing Polychondritis: A rare autoimmune disorder that can cause inflammation and damage to the cartilage in the ears, among other areas, leading to symptoms that might mimic impacted earwax. Its rarity and potential for significant morbidity justify its inclusion in this category.
Otosclerosis: A condition in which there is abnormal bone growth in the middle ear, which can cause hearing loss. While not directly related to impacted earwax, it can present with similar symptoms and is thus considered a rare differential diagnosis.