**Question:** The absence of which of the following makes a diagnosis of pulmonary embolism (PE) unlikely?

**Core Concept:** Pulmonary embolism (PE) is a life-threatening condition caused by the blockage of pulmonary arteries due to blood clots formed elsewhere in the body. Diagnosis relies on the presence of typical symptoms, signs, and investigations findings. The absence of certain factors strengthens the case against PE.

**Why the Correct Answer is Right:** A normal D-dimer level (Option D) is a significant negative predictor of PE, as elevated D-dimer levels are commonly seen in PE cases. The combination of a low D-dimer level and normal CTPA (Option A) and echocardiogram (Option B) are highly specific for ruling out PE.

**Why Each Wrong Option is Incorrect:** Option C (absence of alternative diagnosis) is not relevant for this question as it focuses on the absence of specific laboratory and imaging findings that support PE.

**Clinical Pearl:** A low D-dimer level combined with normal CTPA and echocardiogram are useful diagnostic criteria to rule out PE in a patient with suspected PE. However, D-dimer testing should be interpreted in the context of clinical suspicion, as a normal D-dimer level can be seen in other conditions like trauma, surgery, pregnancy, and inflammation.

**Correct Answer:** D (normal D-dimer level)

**Explanation:** A normal D-dimer level is a highly specific finding for ruling out a pulmonary embolism, as elevated D-dimer levels are commonly observed in patients with PE. The combination of a low D-dimer level and normal CTPA and echocardiogram further strengthens the case against PE, making it a reliable way to rule out PE in a patient with suspected PE. However, D-dimer testing should be interpreted in the context of clinical suspicion, as a normal D-dimer level can be seen in other conditions like trauma, surgery, pregnancy, and inflammation.