**Core Concept**
The preauricular sulcus is a significant anatomical landmark on the external auditory meatus, which is a small groove or depression located anterior to the external auditory canal. It is a vestigial remnant of the first branchial cleft.

**Why the Correct Answer is Right**
The preauricular sulcus is useful for determining the presence of a congenital anomaly known as a preauricular sinus. A preauricular sinus is a small pit or groove located in the preauricular sulcus, which is a remnant of the first branchial cleft. The presence of a preauricular sinus can be indicative of a genetic predisposition or a congenital anomaly. In some cases, preauricular sinuses can be associated with other congenital anomalies, such as renal anomalies or hearing loss.

**Why Each Wrong Option is Incorrect**
**Option A:** The preauricular sulcus is not useful for determining the presence of a middle ear infection, as this would be diagnosed through other clinical and radiological means.
**Option B:** While the preauricular sulcus is an anatomical landmark, it is not useful for determining the presence of a parotid gland tumor, which would be diagnosed through clinical examination and imaging studies.
**Option C:** The preauricular sulcus is not useful for determining the presence of a facial nerve palsy, as this would be diagnosed through clinical examination and electromyography.

**Clinical Pearl / High-Yield Fact**
The preauricular sulcus is a vestigial remnant of the first branchial cleft, which is a small cleft-like structure that forms during embryonic development. A preauricular sinus is a small pit or groove located in the preauricular sulcus, which can be indicative of a congenital anomaly.

**Correct Answer: D.**