**Core Concept**
Thiamine (Vitamin B1) deficiency can lead to Wernicke's encephalopathy, characterized by a classic triad of confusion, ophthalmoplegia (diplopia, nystagmus), and ataxia. This condition is often seen in pregnant women with hyperemesis gravidarum due to inadequate thiamine intake.

**Why the Correct Answer is Right**
The patient's symptoms of diplopia (double vision), nystagmus (involuntary eye movement), and mental confusion are likely due to Wernicke's encephalopathy. Thiamine plays a crucial role in the metabolism of glucose and the synthesis of neurotransmitters in the brain. Deficiency can lead to neuronal damage and the characteristic clinical features. The condition can be precipitated by the increased metabolic demands of pregnancy and the associated hyperemesis gravidarum.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the symptoms described do not match those of preeclampsia, which typically presents with hypertension, proteinuria, and visual disturbances (e.g., scotomas).

**Option B:** This option is incorrect because the symptoms do not align with those of eclampsia, which is characterized by seizures and coma in a woman with preeclampsia.

**Option C:** This option is incorrect because the described symptoms are not typical of gestational diabetes mellitus, which presents with polyuria, polydipsia, and unexplained weight loss.

**Clinical Pearl / High-Yield Fact**
Wernicke's encephalopathy is a medical emergency that requires prompt treatment with thiamine supplementation. Delayed or inadequate treatment can lead to Korsakoff's syndrome, characterized by chronic memory loss and confabulation.

**Correct Answer:** C. Wernicke's encephalopathy.