**Question:** High plasma drug concentration of phenytoin can cause which of the following adverse effects?

**Core Concept:** Phenytoin is an antiepileptic drug that works by inhibiting neuronal excitability and is used to control seizures. It acts on ion channels, particularly the voltage-gated sodium channels, to reduce neuronal electrical activity.

**Why the Correct Answer is Right:** High plasma concentrations of phenytoin can lead to adverse effects due to excessive inhibition of neuronal excitability. The correct answer is D, **Hypotonia (Weakness)**.

**Why Each Wrong Option is Incorrect:**
A. **Hypokalaemia (Low Potassium):** While hypokalaemia can occur with phenytoin, it is not the main adverse effect associated with high plasma concentrations. Hypotonia is more directly related to the drug's action on neuronal excitability.

B. **Hyponatraemia (Low Sodium):** Similarly, hyponatraemia is not the primary adverse effect of high phenytoin concentrations. Hypotonia is the more relevant consequence of excessive neuronal inhibition.

C. **Hypoglycaemia (Low Blood Sugar):** Hypoglycaemia is not the direct consequence of high phenytoin plasma concentrations. Hypotonia is the primary issue caused by increased neuronal inhibition.

**Clinical Pearl:** High plasma concentrations of phenytoin should be monitored to prevent adverse effects. A therapeutic range for phenytoin is generally between 10-20 micrograms/mL, but it may vary based on individual factors, such as age, renal function, and concomitant medications.

**Correct Answer:** Hypotonia (Weakness)