Medicine The single most useful clinical sign of severity of in a person without underlying lung disease Is A Temp more than 38.5degC B Hea rate more than 100/mm C Systolic BP less than 90 mm/Hg D none 💡 Explanation The single most useful clinical sign of severity of in a person without underlying lung disease Is ✓ Correct Answer: C. Systolic BP less than 90 mm/Hg