**Core Concept**
Estrogen therapy in postmenopausal women can have various effects on the body, including increased risks of certain conditions due to its impact on the cardiovascular system, coagulation pathway, and hormone-sensitive tissues. Estrogen can stimulate the proliferation of cells, leading to an increased risk of certain cancers and other conditions.

**Why the Correct Answer is Right**
Estrogen therapy is known to increase the risk of thromboembolic events, such as deep vein thrombosis and pulmonary embolism, due to its effect on the coagulation pathway and increased levels of clotting factors. It also increases the risk of endometrial cancer, especially in women with a history of unopposed estrogen therapy. Additionally, estrogen therapy may increase the risk of gallstones and breast cancer. However, it is not associated with an increased risk of osteoporosis, as estrogen has a protective effect on bone density.

**Why Each Wrong Option is Incorrect**
* **Option A:** Thromboembolic events - Incorrect because estrogen therapy does increase the risk of thromboembolic events.
* **Option B:** Breast cancer - Incorrect because estrogen therapy may increase the risk of breast cancer.
* **Option C:** Gallstones - Incorrect because estrogen therapy may increase the risk of gallstones.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the addition of a progestin to estrogen therapy in postmenopausal women with an intact uterus can help mitigate the risk of endometrial cancer.

**Correct Answer: D. Osteoporosis**