A. Baig, S. Siddiq, M. R. Butt, and S. Salik, “Association of Serum Vitamin D Levels in Acute Exacerbation of COPD”, IJBR, vol. 3, no. 5, pp. 1010–1014, May 2025, doi: 10.70749/ijbr.v3i5.2209.