**Core Concept**
Secretory otitis media, also known as otitis media with effusion (OME), is characterized by the presence of fluid in the middle ear space without signs of acute infection. In adults, it can be caused by various factors, including anatomic abnormalities, Eustachian tube dysfunction, and local or systemic diseases.

**Why the Correct Answer is Right**
Nasopharyngeal carcinoma is a type of head and neck cancer that can obstruct the Eustachian tube, leading to impaired ventilation of the middle ear and the accumulation of fluid. This obstruction can result from direct invasion of the tumor into the Eustachian tube or from the formation of a mass effect that compresses the tube. The impaired Eustachian tube function can cause a negative pressure in the middle ear, leading to the accumulation of fluid. This is a common cause of unilateral secretory otitis media in adults.

**Why Each Wrong Option is Incorrect**
**Option A:** Chronic suppurative otitis media (CSOM) typically presents with signs of acute infection, such as discharge, fever, and hearing loss, and is not a primary cause of secretory otitis media.

**Option C:** Mastoiditis is an infection of the mastoid air cells and is often associated with acute otitis media. It is not a direct cause of secretory otitis media.

**Option D:** A foreign body in the external ear can cause external otitis or swimmer's ear, but it is not a direct cause of secretory otitis media.

**Clinical Pearl / High-Yield Fact**
It is essential to consider nasopharyngeal carcinoma as a potential cause of unilateral secretory otitis media in adults, especially if there is a history of symptoms such as hearing loss, tinnitus, or a sensation of fullness in the ear. A thorough head and neck examination, including a nasopharyngoscopy, may be necessary to evaluate for this condition.

**✓ Correct Answer: B. Nasopharyngeal carcinoma**