Which acts on intracellular receptor:
**Core Concept**
Steroid hormones are a class of lipophilic molecules that act on intracellular receptors to regulate gene expression. These hormones diffuse through the cell membrane and bind to specific intracellular receptors, which then form a complex with other proteins to activate or repress gene transcription.
**Why the Correct Answer is Right**
Steroid hormones, such as cortisol, aldosterone, and estrogen, bind to specific intracellular receptors, known as steroid hormone receptors. These receptors are located in the cytoplasm or nucleus of target cells and are activated by the binding of steroid hormones. The activated receptor complex then translocates to the nucleus, where it regulates the transcription of specific genes by binding to DNA response elements. This process is known as ligand-activated transcriptional regulation.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because non-steroidal hormones, such as catecholamines and polypeptide hormones, act on cell surface receptors rather than intracellular receptors.
**Option B:** This option is incorrect because the statement is too broad and does not specifically refer to intracellular receptors.
**Option C:** This option is incorrect because while some hormones may have both intracellular and extracellular receptors, the primary mechanism of action of this hormone is not through intracellular receptors.
**Clinical Pearl / High-Yield Fact**
Steroid hormones are lipophilic molecules that can diffuse through the cell membrane, allowing them to act on intracellular receptors. This unique mechanism of action is distinct from that of non-steroidal hormones, which act on cell surface receptors.
**Correct Answer: C. Steroid hormones.**