**Core Concept:**
Unilateral conductive hearing loss refers to a decrease in sound transmission through the outer and middle ear structures. This can be caused by various factors, including trauma, inflammation, tumors, or abnormalities of the ear structures. In the context of history of head injury and normal/movable tympanic membrane, the focus is on middle ear structures and their integrity.

**Why the Correct Answer is Right:**
In this scenario, the correct answer is **D** which is "temporal bone fracture." A temporal bone fracture can potentially damage the ossicles (malleus, incus, and stapes), causing damage to the oval and round window, which are essential components of the middle ear responsible for transmitting vibrations to the inner ear. This can lead to conductive hearing loss on the affected side.

**Why Each Wrong Option is Incorrect:**
A) "Eustachian tube obstruction" is likely a cause of conductive hearing loss on both sides, not just one.
B) "Tympanic membrane perforation" is usually associated with perforation and bleeding, not normal/movable membrane.
C) "Middle ear inflammation" usually presents with symptoms like fever, ear discharge, and tympanic membrane redness, which are not observed in this case.

**Clinical Pearl:**
In patients with a history of head injury and unilateral conductive hearing loss, a temporal bone fracture should be considered in the differential diagnosis. This is crucial as early recognition and treatment can prevent complications like cholesteatoma formation and mastoiditis. A thorough clinical examination, including middle ear inspection, is essential in such cases.