**Core Concept**
Sickle cell anemia is a genetic disorder characterized by the production of abnormal hemoglobin (HbS), leading to vaso-occlusive crises, chronic hemolysis, and increased susceptibility to infections. In this context, the patient's presentation with fever, cough, and bilateral diffuse alveolar infiltrates on chest X-ray suggests a pulmonary complication.

**Why the Correct Answer is Right**
The patient's condition is likely a sickle cell pulmonary crisis, which is a known complication of sickle cell disease. This condition is thought to result from the sickling of red blood cells in the pulmonary vasculature, leading to microvascular occlusion and subsequent hypoxemia. The patient's high heart rate (tachycardia) and hypertension (hypertension) are also consistent with this diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Sickle cell pulmonary crisis is a known complication of sickle cell disease, so this statement is true.
**Option B:** The patient's presentation with fever, cough, and bilateral diffuse alveolar infiltrates on chest X-ray is consistent with a pulmonary infection, which is a common complication of sickle cell disease. However, the exact nature of the infection (viral or bacterial) is not specified, so this statement may not be entirely accurate. Nonetheless, it is not the most incorrect statement among the options.
**Option C:** Sickle cell disease is associated with an increased risk of infections, including pneumonia. The patient's presentation is consistent with a pulmonary infection, so this statement is true.
**Option D:** Sickle cell pulmonary crisis is characterized by severe hypoxemia, which can lead to respiratory failure. The patient's SaO2 of 85% is consistent with this diagnosis, so this statement is true.

**Clinical Pearl / High-Yield Fact**
Sickle cell disease is associated with an increased risk of infections, including pneumonia, and sickle cell pulmonary crisis is a known complication of this condition. Patients with sickle cell disease should be evaluated promptly for any signs or symptoms of infection or pulmonary complications.

**Correct Answer:** D.