**Core Concept**
Retrobulbar hemorrhage is a serious condition that can occur in the orbit behind the eyeball. It is often associated with head injuries and can lead to severe complications, including increased intraocular pressure, vision loss, and even death.

**Why the Correct Answer is Right**
The presence of a black eye (periorbital ecchymosis) associated with subconjunctival hemorrhage in a patient with head injury suggests the possibility of a retrobulbar hemorrhage. This occurs when there is a fracture of the orbital floor or medial wall, causing bleeding into the orbit. The blood accumulates behind the eyeball, leading to increased pressure and swelling of the eyelids and conjunctiva. The subconjunctival hemorrhage is a result of the increased pressure causing the blood vessels in the conjunctiva to rupture.

**Why Each Wrong Option is Incorrect**
**Option A:** A subconjunctival hemorrhage can occur due to various reasons, including hypertension, trauma, or systemic coagulopathy, but it is not specifically associated with retrobulbar hemorrhage in the context of head injury.
**Option B:** A black eye can be caused by a blow to the face, but it does not necessarily indicate a retrobulbar hemorrhage, which requires a specific fracture pattern and associated symptoms.
**Option C:** A subconjunctival hemorrhage can be caused by straining or coughing, but it is not a direct indicator of retrobulbar hemorrhage in a patient with head injury.

**Clinical Pearl / High-Yield Fact**
In a patient with head injury, the presence of a black eye associated with subconjunctival hemorrhage should raise suspicion for retrobulbar hemorrhage, a potentially life-threatening condition that requires prompt evaluation and management.

**Correct Answer: D.**