**Core Concept**
Selective estrogen receptor modulators (SERMs) are a class of compounds that act on the estrogen receptor (ER) to produce tissue-specific agonist or antagonist effects. This unique property allows SERMs to mimic the effects of estrogen in some tissues while blocking its effects in others.

**Why the Correct Answer is Right**
SERMs bind to the estrogen receptor, but their affinity and activity can vary depending on the specific receptor isoform and the tissue context. For instance, tamoxifen, a well-known SERM, acts as an estrogen agonist in the uterus and breast, but as an antagonist in the bone and liver. This dual action is due to differences in the conformation of the estrogen receptor and the presence of co-regulatory proteins in various tissues. The unique property of SERMs is their ability to modulate the estrogen receptor in a tissue-specific manner, which is essential for their therapeutic applications in conditions such as breast cancer and osteoporosis.

**Why Each Wrong Option is Incorrect**
**Option A:** SERMs are not known for their anti-androgenic activity, which is a characteristic of some other classes of medications.
**Option B:** While SERMs do have some effects on bone density, their primary mechanism of action is not through the modulation of bone resorption or formation.
**Option C:** SERMs do not have a unique property related to their ability to cross the blood-brain barrier or affect neurotransmitter levels.

**Clinical Pearl / High-Yield Fact**
SERMs can have a paradoxical effect on breast cancer cells, promoting their growth in some cases. This is because SERMs can act as estrogen agonists in certain breast cancer cells, leading to an increase in cell proliferation.

**Correct Answer:** C. They can act as estrogen agonists in some tissues and estrogen antagonists in others.