**Core Concept**
The patient's condition is characterized by a generalized loss of bone mass, likely due to osteoporosis. Osteoporosis is a bone disease that results from an imbalance between bone resorption and bone formation, leading to decreased bone density and increased risk of fractures. The goal of exogenous hormone therapy in this context is to either slow down bone resorption or enhance bone formation.

**Why the Correct Answer is Right**
Estrogen is a sex hormone that plays a crucial role in maintaining bone health by regulating osteoclast and osteoblast activity. Osteoclasts are responsible for bone resorption, while osteoblasts are involved in bone formation. Estrogen therapy can help balance this process by inhibiting osteoclast activity and promoting osteoblast function, thereby slowing down bone loss and increasing bone density. This is particularly relevant in postmenopausal women, who experience a decline in estrogen levels, leading to an increased risk of osteoporosis.

**Why Each Wrong Option is Incorrect**
**Option A:** While testosterone is an androgen that plays a role in bone health, its effects are less pronounced in women, and it is not the most effective hormone for preventing osteoporosis in postmenopausal women.

**Option B:** Parathyroid hormone (PTH) actually promotes bone resorption by stimulating osteoclast activity, which would exacerbate the patient's bone disease.

**Option C:** Growth hormone (GH) can stimulate bone growth and increase bone density, but its effects are more pronounced in children and adolescents, and it is not typically used to treat osteoporosis in adults.

**Clinical Pearl / High-Yield Fact**
The World Health Organization (WHO) defines osteoporosis as a bone mineral density (BMD) T-score of -2.5 or lower, indicating a significant loss of bone mass.

**Correct Answer:** B. Estrogen