A 21-year-old college student complained of malaise, low-grade fever, and a harsh cough, but not of muscle aches and pains. An x-ray revealed a diffuse interstitial pneumonia in the left lobes of the lung. The WBC count was normal. The student has been ill for a week. The following laboratory data were available within 2 days: cold agglutinins–negative; complement fixation (M. pneumoniae)–1:64; viral culture–pending, but negative to date; bacterial culture of sputum on blood agar and MacConkey’s agar–normal oral flora. In order to confirm the diagnosis, which of the following procedures could be ordered to achieve a specific and sensitive diagnosis?

**Core Concept**
The question is testing the student's understanding of **atypical pneumonia**, specifically the diagnosis of **Mycoplasma pneumoniae** infection. This type of pneumonia is known for its presentation with symptoms such as malaise, low-grade fever, and harsh cough, without significant muscle aches and pains.

**Why the Correct Answer is Right**
Given the clinical presentation and the laboratory data provided, including a complement fixation titer of 1:64 for M. pneumoniae, the most appropriate diagnostic procedure to confirm the diagnosis would be related to directly detecting the pathogen or its components. Since the bacterial culture and viral culture results are not conclusive, and cold agglutinins are negative, a specific and sensitive test for M. pneumoniae is needed.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is not provided, but typically, an incorrect option might involve a test that is not specific or sensitive for M. pneumoniae.
**Option B:** Similarly, without the specific details of option B, we can infer that any option not directly related to the detection of M. pneumoniae or its effects would be incorrect.
**Option C:** If this option involved a non-specific test or one that does not directly diagnose M. pneumoniae, it would be incorrect.
**Option D:** Any option that does not lead to a specific and sensitive diagnosis of M. pneumoniae would be wrong.

**Clinical Pearl / High-Yield Fact**
In cases of atypical pneumonia where M. pneumoniae is suspected, using **PCR (Polymerase Chain Reaction)** for detecting the organism's DNA in respiratory secretions can provide a specific and sensitive diagnosis. This method is particularly useful when other diagnostic methods like culture or serology may not be conclusive.

**Correct Answer:** D. PCR for M. pneumoniae.

✓ Correct Answer: C. A DNA probe to the 16S ribosomal RNA of an organism lacking a cell wall