A person taking an oral sulfonamide is found to have a markedly decreased peripheral blood neutrophil count, but the numbers of platelets and erythrocytes are normal. If the peripheral neutropenia is the result of antineutrophil antibodies being produced in response to taking the sulfonamide, then this patient would be expected to have

**Core Concept**
The patient's peripheral neutropenia is caused by the production of antineutrophil antibodies, which are directed against neutrophils. This is a classic example of an immune-mediated drug reaction, specifically an autoimmune hemolytic anemia or more accurately, an autoimmune neutropenia.

**Why the Correct Answer is Right**
The production of antineutrophil antibodies leads to the activation of the complement system, which causes neutrophils to be destroyed in the spleen. This results in a decrease in the peripheral blood neutrophil count. The mechanism is similar to that of autoimmune hemolytic anemia, where antibodies are directed against red blood cells, leading to their destruction.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not directly related to the scenario described. The production of antineutrophil antibodies does not typically lead to an increase in platelet count.
* **Option B:** This option is incorrect because the production of antineutrophil antibodies does not typically lead to an increase in erythropoietin production, which would stimulate red blood cell production.
* **Option C:** This option is incorrect because the production of antineutrophil antibodies does not typically lead to a decrease in neutrophil production in the bone marrow.

**Clinical Pearl / High-Yield Fact**
Autoimmune neutropenia is a rare but serious side effect of certain medications, including sulfonamides. It is essential to monitor patients taking these medications for signs of neutropenia and to discontinue the medication if neutropenia develops.

**Correct Answer:** D

✓ Correct Answer: D. Hyperplasia of the bone marrow myeloid series