Osteoporosis, an age-related disease of the skeleton system, may occur due to:
Decreased level of estrogen
Accumulation of uric acid leading to inflammation of joints
Immune disorder affecting neuro-muscular junction leading to fatigue
High concentration of Ca++ and Na+

Estrogen regulates osteoblast activity and stimulates bone formation. Menopause leads to lower estrogen levels, causing lower/no osteoblast activity. The overall effect is lower bone mass and thinning of bones, which are more prone to fractures. Accumulation of uric acid leading to inflammation of joints leads to gout. Myasthenia gravis is an immune disorder of the neuromuscular junction that causes weakening of muscles. Therefore, the correct answer is option C (Decreased level of estrogen).