**Core Concept**
Infantile spasms, also known as West syndrome, is a severe and often intractable form of epilepsy that affects infants. The treatment involves medications that modulate the excitatory-inhibitory balance in the brain, particularly targeting the GABAergic and glutamatergic systems.

**Why the Correct Answer is Right**
The drug of choice for infantile spasms is ACTH (Adrenocorticotropic hormone) because it acts on the GABAergic system by increasing the expression of GABA receptors and enhancing GABAergic transmission. This leads to a decrease in neuronal excitability and an increase in seizure threshold. ACTH also has a corticosteroid-like effect, which further contributes to its anticonvulsant action.

**Why Each Wrong Option is Incorrect**
**Option A:** Vigabatrin is an antiepileptic medication that works by irreversibly inhibiting GABA transaminase, thereby increasing GABA levels. However, it is not the first-line treatment for infantile spasms.
**Option B:** Topiramate is a broad-spectrum antiepileptic drug that targets multiple ion channels and receptors. While it can be effective for various seizure types, it is not the preferred treatment for infantile spasms.
**Option C:** Diazepam is a benzodiazepine that enhances GABAergic transmission by facilitating GABA receptor binding. However, it is not typically used as a first-line treatment for infantile spasms due to its potential for tolerance and dependence.

**Clinical Pearl / High-Yield Fact**
Infantile spasms often present with a characteristic "salaam" or "jackknife" posture, and the EEG typically shows a hypsarrhythmic pattern. Early recognition and treatment of infantile spasms are crucial to prevent long-term cognitive and developmental impairment.

**Correct Answer: A. ACTH**