**Core Concept**
The infant's presentation of irritability, lethargy, and a generalized tonic-clonic seizure following a minor trauma suggests a possible diagnosis of subdural hematoma or increased intracranial pressure due to a skull fracture. This condition is a medical emergency requiring prompt diagnosis and treatment.

**Why the Correct Answer is Right**
The correct answer is a CT scan of the head. This is because the infant's presentation of irritability, lethargy, and seizures following a minor trauma raises concern for a possible skull fracture or subdural hematoma. A CT scan of the head is the most appropriate diagnostic study in this scenario as it quickly and accurately provides information about the presence and severity of any intracranial injuries.

* A CT scan is the preferred imaging modality in this scenario due to its rapid acquisition time and ability to detect acute hemorrhage.
* The CT scan will help identify any skull fractures, subdural hematomas, or other intracranial injuries that may be contributing to the infant's symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** Skull X-ray
A skull X-ray is not the most appropriate diagnostic study in this scenario as it may not detect subdural hematomas or other intracranial injuries.

**Option B:** MRI of the head
While an MRI of the head may be helpful in evaluating the infant's symptoms, it is not the most appropriate initial diagnostic study due to its longer acquisition time and lower availability compared to a CT scan.

**Option C:** Ultrasound of the head
An ultrasound of the head is not a suitable diagnostic study for evaluating intracranial injuries in this scenario.

**Clinical Pearl / High-Yield Fact**
In infants and young children, a minor trauma can sometimes lead to significant intracranial injuries, such as skull fractures or subdural hematomas. A high index of suspicion and prompt imaging with a CT scan are essential in diagnosing and treating these conditions.

**Correct Answer:** C.