Gram’s stain of a smear from a sputum sample demonstrates gram-positive lancet-shaped diplococci in sho chains. Which of the following would most likely be identified after culturing?
**Core Concept**
The presence of gram-positive lancet-shaped diplococci in short chains on a Gram stain suggests a specific bacterial morphology, which can be used to narrow down the differential diagnosis. This particular morphology is characteristic of certain species of *Streptococcus*.
**Why the Correct Answer is Right**
The gram-positive lancet-shaped diplococci in short chains are a classic description of *Streptococcus pneumoniae*, also known as pneumococcus. This bacterium is a common cause of pneumonia, meningitis, and otitis media. The short chain morphology is a key identifying feature of this organism. *S. pneumoniae* is a capsulated bacterium, and the capsular polysaccharide is a major virulence factor.
**Why Each Wrong Option is Incorrect**
**Option A:** *Staphylococcus aureus* is a gram-positive bacterium, but it typically forms clusters or tetrads, not short chains. *S. aureus* also has a different morphology and is not commonly associated with the described Gram stain pattern.
**Option B:** *Haemophilus influenzae* is a gram-negative bacterium that typically forms small, pleomorphic coccobacilli, not lancet-shaped diplococci.
**Option C:** *Moraxella catarrhalis* is a gram-negative bacterium that typically forms small, pleomorphic cocci, not lancet-shaped diplococci.
**Clinical Pearl / High-Yield Fact**
*Streptococcus pneumoniae* is a key cause of bacterial pneumonia, particularly in elderly or immunocompromised patients. It is also a common cause of community-acquired pneumonia and is often associated with a high mortality rate if left untreated.
**Correct Answer:** C. *Streptococcus pneumoniae*.