**Core Concept**
Rickettsial infections are a group of tick-borne and flea-borne illnesses caused by the Rickettsia genus, leading to severe systemic complications. The pathogenesis of rickettsial infections involves the invasion of endothelial cells and the subsequent induction of inflammation, which can lead to organ dysfunction.

**Why the Correct Answer is Right**
The primary cause of mortality in rickettsial infections is the development of acute respiratory distress syndrome (ARDS) and acute kidney injury (AKI). This is due to the systemic inflammatory response caused by the infection, leading to endothelial damage and the release of pro-inflammatory cytokines. The resulting hypoxemia and renal failure can be fatal if not promptly treated. The pathophysiology of rickettsial infections involves the activation of various immune cells, including neutrophils and macrophages, which release reactive oxygen species and other inflammatory mediators that contribute to tissue damage.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not specified, so it cannot be evaluated.
**Option B:** While sepsis is a possible complication of rickettsial infections, it is not the primary cause of mortality.
**Option C:** Cardiac failure is not a direct consequence of rickettsial infections.

**Clinical Pearl / High-Yield Fact**
Rickettsial infections can be differentiated from other tick-borne illnesses by the presence of a characteristic petechial rash, which is a result of the infection's effect on the endothelial cells.

**Correct Answer: D. The development of acute respiratory distress syndrome (ARDS) and acute kidney injury (AKI).**