## Core Concept
The question tests the understanding of seizure etiology in adults, particularly those presenting with a first-time generalized tonic-clonic seizure. The differential diagnosis for seizures in adults includes various causes such as electrolyte imbalances, structural brain lesions, infections, and metabolic disorders.

## Why the Correct Answer is Right
Idiopathic or primary epilepsy typically presents earlier in life, often with a family history. Given that this 60-year-old man has had no previous seizures and no other medical illnesses, a new-onset seizure at this age makes an **idiopathic/genetic epilepsy** less likely. Idiopathic epilepsies usually manifest during childhood or adolescence.

## Why Each Wrong Option is Incorrect
- **Option A:** **Metabolic disturbances** (e.g., hyponatremia, hypoglycemia, uremia) can cause seizures at any age and are a plausible cause in this scenario, especially without a prior history of seizures.
- **Option B:** **Structural brain lesions** (e.g., tumors, strokes) are a common cause of new-onset seizures in adults and can present at any age, making this a likely consideration.
- **Option C:** **Infections** (e.g., meningitis, encephalitis) can cause seizures and are a possibility, especially if there's suspicion of an infectious process.

## Clinical Pearl / High-Yield Fact
A key point to remember is that new-onset seizures in adults necessitate an evaluation for structural, metabolic, or infectious causes. Idiopathic epilepsy presenting de novo in adulthood is extremely rare. Therefore, investigations such as imaging (e.g., MRI), EEG, and basic metabolic workup are crucial.

## Correct Answer: D. Idiopathic/genetic epilepsy.