Steroid hormones exert their main action in target cells by which mechanism?

Introduction / Context:
Hormones can be broadly divided into water soluble and lipid soluble types, and they act through different mechanisms. Steroid hormones such as cortisol, aldosterone, estrogen, progesterone, and testosterone are lipid soluble and are derived from cholesterol. Understanding how these hormones work at the cellular level is fundamental in physiology, pharmacology, and clinical endocrinology. This question tests your understanding of the classic mechanism of action of steroid hormones inside target cells.

Given Data / Assumptions:
• The question is specifically about steroid hormones, which are lipid soluble. • Options mention mitochondrial DNA activation, membrane receptors, hypothalamic regulation, and nuclear gene expression. • Standard physiology of hormone action is assumed, without rare or special cases.

Concept / Approach:
Because steroid hormones are lipid soluble, they can pass directly through the phospholipid bilayer of the cell membrane. Inside the cell, they bind to specific intracellular receptors located either in the cytoplasm or within the nucleus. The hormone receptor complex then binds to DNA and influences transcription of particular genes. This alters protein synthesis and produces relatively slow but long lasting effects. Therefore, the correct mechanism must describe entry into the cell, binding to an intracellular receptor, and modification of gene expression in the nucleus.

Step-by-Step Solution:
Step 1: Recall that steroid hormones are lipid soluble and can diffuse across the cell membrane. Step 2: Inside the cell, they combine with specific receptor proteins in the cytoplasm or nucleus. Step 3: The hormone receptor complex then interacts with DNA at hormone response elements, altering transcription of target genes. Step 4: Changed gene transcription leads to altered protein synthesis, which produces the hormone's effects. Step 5: Match this mechanism with the option that explicitly mentions entering the cell, binding an intracellular receptor, and altering gene expression in the nucleus.

Verification / Alternative check:
An easy way to verify is to contrast steroid hormones with peptide hormones. Peptide hormones, such as insulin, are water soluble and bind to receptors on the cell surface, using second messengers. Steroid hormones do not primarily use this pathway; instead, they follow a genomic mechanism through nuclear receptors. Only the option that mentions entry into the cell and gene expression changes fits this well known pattern, confirming it as correct.

Why Other Options Are Wrong:
Option A is incorrect because mitochondrial DNA is not the main target of steroid hormone action in standard physiology. Option B is incorrect because steroid hormones do not rely only on cell surface receptors; that mechanism is typical of many peptide hormones. Option C is incorrect because although the hypothalamus regulates hormone release, the direct action of steroid hormones occurs in target tissues at the level of gene expression.

Common Pitfalls:
Students often confuse the mechanisms of different hormone classes and may wrongly assume that all hormones work through membrane receptors and second messenger systems. Another mistake is to overlook the importance of gene transcription in long term hormonal effects. Always remember that steroid hormones are lipid soluble, act through intracellular receptors, and primarily alter gene expression in the nucleus.

Final Answer:
Steroid hormones mainly act by entering the cell, binding an intracellular receptor, and altering gene expression in the nucleus.