A 36-year-old woman has a cough and fever for 1 week. On physical examination, her temperature is 38.3deg C. She has diffuse crackles in all lung fields. A chest radiograph shows bilateral extensive infiltrates. CBC shows hemoglobin, 13.9 g/ dL; hematocrit, 42%; MCV, 89 mm3; platelet count, 210,000/mm3; and WBC count, 56,000/mm3 with 63% segmented neutrophils, 16% bands, 7% metamyelocytes, 3% myelocytes, 1% blasts, 8% lymphocytes, and 2% monocytes. A bone marrow biopsy is obtained and shows normal maturation of myeloid cells. Which of the following is the most likely diagnosis?

**Core Concept**
The patient's presentation of fever, cough, and diffuse crackles in all lung fields, along with a chest radiograph showing bilateral extensive infiltrates, suggests a severe and potentially life-threatening infection. The elevated WBC count with a predominance of immature granulocytes (bands, metamyelocytes, and myelocytes) indicates a bone marrow response to an infection or inflammation.

**Why the Correct Answer is Right**
The patient's bone marrow biopsy shows normal maturation of myeloid cells, which rules out a primary bone marrow disorder. The presence of bilateral extensive infiltrates on the chest radiograph and the patient's clinical presentation are consistent with a severe bacterial pneumonia. The elevated WBC count with a predominance of immature granulocytes is a classic response to bacterial infections, as it indicates a bone marrow response to an increased demand for white blood cells. The normal bone marrow maturation pattern suggests that the bone marrow is functioning normally and is able to respond to the infection.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the patient's presentation of a severe infection is not consistent with a primary bone marrow disorder, such as leukemia, which would typically show abnormal maturation of myeloid cells on a bone marrow biopsy.

**Option B:** This option is incorrect because the patient's WBC count is elevated, indicating an infection or inflammation, rather than a decrease in WBC count, which would be seen in a case of severe bone marrow failure.

**Option C:** This option is incorrect because the patient's presentation of a severe infection is not consistent with a chronic infection, such as tuberculosis, which would typically show a different pattern of WBC count and bone marrow biopsy findings.

**Option D:** This option is incorrect because the patient's presentation of a severe infection is not consistent with a primary lung disorder, such as chronic obstructive pulmonary disease (COPD), which would typically show a different pattern of WBC count and chest radiograph findings.

**Clinical Pearl / High-Yield Fact**
In cases of severe bacterial infections, the bone marrow responds by releasing immature granulocytes into the bloodstream, which can lead to a marked increase in WBC count. This is a classic response to bacterial infections and is often seen in patients with severe pneumonia.

**Correct Answer:** C.

✓ Correct Answer: D. Leukemoid reaction