What should be administered if a patient appears malnourished?

When a patient appears malnourished, the administration of thiamine is particularly important due to the risk of Wernicke's encephalopathy, which is a serious neurological condition associated with thiamine deficiency. This deficiency is often seen in individuals with malnutrition, especially those who are alcohol dependent or have other conditions leading to poor nutritional intake. Thiamine plays a critical role in carbohydrate metabolism and energy production, and its deficiency can lead to severe cognitive and motor impairments.

While other options like glucose, vitamin C, and iron supplements have their own roles in nutrition and health, they do not directly address the immediate risk of complications associated with malnutrition in the same way thiamine does. For instance, glucose can provide energy but might not resolve the underlying vitamin deficiencies, and iron is essential for addressing anemia but does not help with neurological function. Therefore, thiamine is the correct choice in this context, as it specifically targets the potential neurological issues that can arise from malnutrition.