**Core Concept**
Sustained neutropenia refers to a prolonged decrease in the production of neutrophils, a type of white blood cell essential for fighting infections. This condition can be caused by various factors, including medications, infections, and bone marrow disorders.

**Why the Correct Answer is Right**
The correct answer is **C. Chloramphenicol**. Chloramphenicol is a broad-spectrum antibiotic that can cause sustained neutropenia as a side effect. This is because chloramphenicol inhibits the production of neutrophils by suppressing the activity of the bone marrow. Specifically, chloramphenicol can inhibit the production of neutrophils by affecting the activity of the enzyme inosine monophosphate dehydrogenase (IMPDH), which is crucial for the production of guanosine triphosphate (GTP) in the bone marrow.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because azithromycin is a macrolide antibiotic that is less likely to cause sustained neutropenia compared to chloramphenicol. While azithromycin can cause neutropenia, it is generally a rare side effect.

**Option B:** This option is incorrect because amoxicillin is a penicillin antibiotic that is not typically associated with sustained neutropenia. Amoxicillin may cause neutropenia, but it is generally a mild and transient side effect.

**Option C:** This option is correct because chloramphenicol is a well-known cause of sustained neutropenia.

**Option D:** This option is incorrect because ciprofloxacin is a fluoroquinolone antibiotic that can cause neutropenia, but it is generally a rare side effect and not as severe as chloramphenicol.

**Clinical Pearl / High-Yield Fact**
Chloramphenicol-induced neutropenia is a well-known side effect of this antibiotic, and it is essential to monitor patients closely for signs of neutropenia while they are taking chloramphenicol.

**Correct Answer:** C. Chloramphenicol