The most likely cardiac rhythm on this patient's ECG is atrial fibrillation. 

Atrial fibrillation is a tachyarrhythmia that is characterized on ECG by absence of distinct P waves, oscillating "f" waves that cause an irregular baseline rhythm, and abnormal, inconsistent R-R intervals that produce an irregularly irregular rhythm.

When a patient is in atrial fibrillation, the patient may be asymptomatic, but at other times, the patient may complain of a rapid heartbeat, or a feeling of uneasiness. The clues in this case that the patient is in atrial fibrillation are that the ECG shows the characteristic irregularly irregular rhythm, with an absence of any distinct P waves.

Clinically, the other clues are that the patient is complaining of palpitations while simultaneously experiencing sudden onset of severe, diffuse abdominal pain that is out of proportion to the abdominal physical exam, and is accompanied by rectal bleeding. This is very consistent with mesenteric ischemia (a condition in which a patient, typically in atrial fibrillation, projects a blood clot to one of the mesenteric vessels, causing ischemia to the bowel served by the affected vessel). While, this was not asked in the question explicitly, and the question could be answered without this knowledge, this helps confirm the diagnosis if you are aware of it.

The other answers are incorrect for the following reasons:

1) Atrial flutter typically presents with a "sawtooth" waveform and has more regularity in wavelength. 

2) Sick sinus syndrome is also known as tachy-brady syndrome, indicating that the patient has frequent fluctuations between tachycardia and bradycardia. In this instance, we have ECG findings that are very consistent with atrial fibrillation and show no evidence of bradycardia.

3) First-degree heart block and complete heart block are each typically characterized by bradycardia rather than tachycardia, and do not appear like atrial fibrillation on ECG.

Substernal chest pain that radiates to the jaw is classic of myocardial infarction. Elevated troponin and EKG changes help confirm the diagnosis (reference levels are about . ST segment elevation is classically seen in myocardial infarctions that result in positive troponin.

The correct answer is "hypothermia." Osborn waves on electrocardiogram are the pathognomonic finding associated with hypothermia (to a core body temperature of lower than 32 C). Osborn waves are observed as upward deflections in the ECG reading between the QRS and ST intervals with an elevation at the S point (which is sometimes also referred to as the J point). While hypothermia is the most common cause of Osborn waves, they may also be observed in patients with coronary vasospasm, hypercalcemia, ventricular fibrillation, or brain trauma. 

Neither hyperthermia, ethanol toxicity, carbon monoxide poisoning, nor lead poisoning are associated with Osborn waves on ECG.

Peaked T-waves are a warning sign for elevated levels of potassium in the body. On the other hand, flattened T-waves are potentially indicative of low levels of potassium. Hyperkalemia (high potassium) is a potentially dangerous condition that can lead to cardiac abnormalities and potentially death.

Sinus tachycardia is the most common EKG finding in a pulmonary embolism. This represents the heart beating faster as an infarction occurs within the lung tissue. Peaked T waves are seen in hyperkalemia. ST elevation is seen in myocardial infarction, and ST depression is seen in myocardial ischemia. Sinus bradycardia would not be the expected finding in a PE.