Clinical reasoning describes cognitive process involved in medical decision-making. This process requires effective cognitive skill to arrive to a final diagnosis through a series of inferences derived from medical histories, physical examination findings as well as laboratory data. Effective decision making is very important in emergency department which is loaded with the highest decision densities as well as diagnostic uncertainty among all other medical fields. Emergency physicians are not only facing the physical challenges of doing long hour, demanding shift work but also mental challenges of high cognitive load. Various clinical decision making strategies have been reported by literatures but no research has yet to define the prevalence of any of them. Using a real case scenario of a 34 years old Sarawakian lady presented with respiratory distress to Emergency & Trauma Department of Hospital Sultanah Aminah Johor Bahru who eventually succumbed to endotracheal intubation with a final diagnosis of advanced retroviral disease with pneumocystis pneumonia (PCP), various types of clinical decision making strategies are being discussed in different clinical phases before arriving to the final diagnosis. The advantages and disadvantages of each method are also being highlighted. In conclusion, there should be increase emphasize on learning and teaching decision making strategies and it’s underlying cognitive phenomena to help emergency physicians’ clinical decision process become more effective and less error prone.