Which one of the following pairs of chemical substances, is correctly categorised?

Calcitonin and thymosin - Thyroid hormones

Pepsin and prolactin - Two digestive enzymes secreted in stomach

Troponin and myosin - Complex proteins in striated muscles

Secretin and rhodopsin - Polypeptide hormones

Answer

Troponin and myosin - Complex proteins in striated muscles

Striated muscle consists of large number of muscle fibres called myofibrils. Myofibrils are made up of actin and myosin protein. The actin or secondary myofilaments also possess troponin and tropomyosin protein.
Thymosin hormone is secreted from thymus gland. Prolactin is a hormone secreted from anterior lobe of pituitary.
Rhodopsin is a pigment found in rod cells of eye.