Ubaid Ullah Sami, Saeed Ur Rehman, Adnan Ghafoor, Zayed Naser, Saleh Ahmad, Hafeez Ud Din, “Serum Vitamin D Level in Chronic Obstructive Pulmonary Disease and its Relation with Severity”, Pakistan Journal of Medical & Health Sciences, vol. 16, no. 09, p. 941, Nov. 2022.