This patient most likely has obstructive shock from a hemodynamically significant acute pulmonary embolism (PE). Patients with malignancy are at increased risk of thrombosis and embolic events. Obstructive shock is a subset of cardiogenic shock in which there is mechanical blockage in the central circulation, as can occur with massive PE (as in this patient) or tension pneumothorax. The findings that support obstructive shock due to PE include hypotension, an accentuated P₂, and profound hypoxemia. With hemodynamically significant PE, the electrocardiogram (ECG) may show the classic S1Q3T3 pattern caused by right ventricular strain due to acute obstruction of the pulmonary artery, but this is not a sensitive finding for PE and its absence does not exclude the diagnosis. Thrombolytic therapy or surgical thrombectomy are therapeutic considerations in patients with massive PE causing shock.

Anaphylactic shock is a type of distributive shock, as might occur if a patient with an allergy to penicillin were given either penicillin or a related agent to which she reacted. Anaphylaxis is an IgE-mediated reaction and manifests within minutes to 1 hour after exposure to the implicated antigen. Ninety percent of patients have a cutaneous reaction such as urticaria, angioedema, or flushing. Bronchospasm and wheezing are common. This patient received meropenem 2 days before developing shock and has no evidence of skin changes or wheezing, making anaphylaxis an unlikely diagnosis.

Acute myocardial infarction could lead to sudden deterioration in clinical status based on a rapid decrease in left ventricular output. However, more specific changes associated with significant myocardial ischemia would be expected on the ECG. Additionally, the severe hypoxia in the setting of acute myocardial infarction is usually caused by inadequate cardiac output, and this patient's clear lungs on auscultation are not consistent with this diagnosis. Septic shock is a type of distributive shock and could account for the hypotension seen in this patient, but the sudden appearance of hypotension and hypoxemia in a patient whose clinical condition was improving is not typical of septic shock.