Which of the following is a disorder of the muscular/skeletal system?

Osteoporosis,an age-related disease of the skeletal system,may occur due to:

Which of the following is an incorrect statement about myasthenia gravis?
$(I)$ It is not an autoimmune disorder.
$(II)$ It affects the neuromuscular junction.
$(III)$ It occurs due to low $Ca^{++}$ in body fluid.
$(IV)$ Paralysis of skeletal muscle is a symptom of myasthenia gravis.

Decreased level of oestrogen in the human body leads to:

Select the correct statement with respect to disorders of muscles in humans.

Assertion : Arthritis or inflammation of a joint makes the joint painful.
Reason : Some toxic substances are deposited at the joint.