**Core Concept**
The patient presents with hemoptysis, which is the coughing up of blood or blood-stained mucus, and a polypoid lesion obstructing the left lower lobe orifice. This clinical presentation, combined with the radiographic findings, suggests a diagnosis of a vascular lesion within the lung.

**Why the Correct Answer is Right**
The patient's symptoms and findings are consistent with a pulmonary arteriovenous malformation (PAVM). A PAVM is an abnormal connection between the pulmonary arteries and veins, which can cause hemoptysis due to the high-pressure blood flow through the malformation. The polypoid lesion seen on bronchoscopy is likely a dilated pulmonary artery or vein within the malformation. The negative bronchial washings and biopsy results suggest that the lesion is not a malignancy.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because a pulmonary embolism typically presents with sudden onset of dyspnea, chest pain, and possibly hemoptysis, but the patient's symptoms have been intermittent over 5 weeks.
* **Option B:** This option is incorrect because bronchiectasis is characterized by dilated bronchi and chronic sputum production, which is not present in this patient.
* **Option C:** This option is incorrect because a lung abscess would typically present with fever, sputum production, and possibly a focal infiltrate on chest x-ray.

**Clinical Pearl / High-Yield Fact**
PAVMs are often associated with hereditary hemorrhagic telangiectasia (HHT), a genetic disorder that can cause multiple PAVMs. Patients with HHT may present with recurrent hemoptysis, and the diagnosis can be confirmed by imaging studies or genetic testing.

**Correct Answer:** C. Pulmonary arteriovenous malformation.