Gastrointestinal Bleed with Possible Intussusception or Volvulus: This diagnosis could explain the black stool (melena) due to upper GI bleeding, and the right lower quadrant pain could be due to intussusception or volvulus, which are conditions where a part of the intestine telescopes into another or twists, respectively. Throat tightness might be unrelated or could be a referred symptom in a patient under significant stress or pain.

Peptic Ulcer Disease with Bleeding: This could cause the black stool due to bleeding from an ulcer in the stomach or duodenum. The right lower quadrant pain might be a separate issue, such as appendicitis or a musculoskeletal problem, but could also be referred pain. Throat tightness could be anxiety-related.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis could cause abdominal pain, including in the right lower quadrant, and could potentially lead to bleeding, resulting in black stools. Throat tightness might be a symptom of overall distress or a separate condition.

Appendicitis: Although the pain is typically in the right lower quadrant, appendicitis can sometimes present with atypical symptoms. Missing this diagnosis could lead to perforation and severe consequences. Black stool could be from a separate issue, and throat tightness might be unrelated.
Ectopic Pregnancy: In women of childbearing age, ectopic pregnancy is a critical diagnosis not to miss. It can cause abdominal pain and potentially some gastrointestinal symptoms if there's associated bleeding. Throat tightness is less directly related but could be part of a systemic response to severe pain or stress.
Mesenteric Ischemia: This condition involves insufficient blood flow to the intestines and can cause severe abdominal pain, which might be localized to the right lower quadrant. It's less likely to directly cause throat tightness or black stools but is critical to diagnose early due to its high mortality rate.

Henoch-Schönlein Purpura (HSP): A systemic vasculitis that can cause abdominal pain, gastrointestinal bleeding (leading to black stools), and potentially throat symptoms if there's upper respiratory involvement. It's more common in children but can occur in adults.
Wegener's Granulomatosis (Granulomatosis with Polyangiitis): A rare autoimmune disorder that can affect multiple systems, including the respiratory tract (causing throat symptoms), gastrointestinal system (causing abdominal pain and potentially black stools), and kidneys. It's a diagnosis that requires a high index of suspicion.