## **Core Concept**
The question tests knowledge of a specific bacterial infection characterized by a distinctive staining pattern in a patient with a history of coughing. The key detail here is the bipolar staining pattern of the organism in the sputum sample. This pattern is classically associated with *Bordetella pertussis*, the causative agent of whooping cough or pertussis.

## **Why the Correct Answer is Right**
The correct answer, *Bordetella pertussis*, is right because it is known for causing pertussis or whooping cough, which presents with bouts of coughing. A hallmark laboratory feature of *B. pertussis* is its appearance on Gram stain, which shows small, Gram-negative coccobacilli with a characteristic bipolar staining, often described as resembling "closed fists" or "eyebrows". This staining pattern is due to the uneven distribution of the bacterial cell's staining material.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Streptococcus pneumoniae* does not exhibit bipolar staining. It is a Gram-positive diplococcus and is not typically associated with the clinical presentation described.
- **Option B:** *Haemophilus influenzae* is a Gram-negative coccobacillus but does not characteristically show bipolar staining. It can cause respiratory infections but is not specifically identified by this staining pattern.
- **Option D:** *Klebsiella pneumoniae* is a Gram-negative rod that can cause pneumonia but does not exhibit the bipolar staining characteristic of *B. pertussis*.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that *Bordetella pertussis* infections often occur in partially immunized or unimmunized children, presenting with a characteristic "whoop" following bouts of coughing. The diagnosis can be confirmed through PCR or culture, but the distinctive bipolar staining pattern on Gram stain is highly suggestive.

## **Correct Answer:** .