**Core Concept**
Vitamins can be classified into two categories based on their mechanism of action: vitamins that act on intracellular receptors and those that act on intracellular enzymes. Vitamins acting on intracellular receptors, also known as nuclear receptors, modulate gene expression by binding to specific DNA sequences. These receptors play a crucial role in regulating various physiological processes, including metabolism, growth, and development.

**Why the Correct Answer is Right**
Steroid hormones, such as cortisol and aldosterone, are a classic example of vitamins acting on intranuclear receptors. These hormones bind to specific nuclear receptors, which then form a complex with other proteins to regulate the expression of target genes. The transcriptional activity of these receptors is mediated by the binding of the hormone-receptor complex to specific DNA sequences, known as hormone response elements. This results in the activation or repression of gene expression, leading to various physiological effects.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because vitamins acting on intracellular enzymes, such as thiamine and riboflavin, do not bind to intranuclear receptors.

**Option B:** This option is incorrect because non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, do not act on intranuclear receptors.

**Option C:** This option is incorrect because certain neurotransmitters, such as serotonin and dopamine, do not bind to intranuclear receptors.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that vitamins acting on intranuclear receptors, such as steroid hormones, have a unique mechanism of action that involves gene regulation. This is in contrast to vitamins acting on intracellular enzymes, which directly participate in biochemical reactions.

**Correct Answer: D. Steroid hormones**