Appendicitis: The presentation of diffuse tenderness with prominent right lower quadrant tenderness and rebound tenderness is highly suggestive of appendicitis, especially if the pain initially started around the umbilicus and then localized to the right lower quadrant.

Gastroenteritis: Inflammation of the stomach and intestines can cause diffuse tenderness and rebound tenderness, although the presence of prominent right lower quadrant tenderness might be less common.
Mesenteric Lymphadenitis: This condition, often seen in children and young adults, involves inflammation of the lymph nodes in the mesentery and can mimic appendicitis in its presentation.
Ovarian Torsion or Ovarian Cyst: In females, these conditions can cause severe lower abdominal pain, often localized to one side, and might be associated with tenderness and rebound tenderness.

Ectopic Pregnancy: Although less common, an ectopic pregnancy can present with abdominal pain and tenderness, including rebound tenderness, and is a medical emergency.
Intestinal Obstruction: This condition can cause severe abdominal pain, tenderness, and rebound tenderness, especially if there is associated ischemia or perforation.
Diverticulitis: Inflammation of a diverticulum in the intestine can cause severe pain, usually in the left lower quadrant but can be atypical, and rebound tenderness.
Perforated Ulcer: A perforated peptic ulcer can cause sudden onset of severe abdominal pain and rebound tenderness due to peritonitis.

Meckel's Diverticulitis: Inflammation of a Meckel's diverticulum, a congenital anomaly of the small intestine, can mimic appendicitis.
Inflammatory Bowel Disease (IBD) Flare: Conditions like Crohn's disease or ulcerative colitis can cause abdominal pain and tenderness, although the presentation is usually more chronic.
Ruptured Ovarian Cyst: While not as immediately life-threatening as some other conditions, a ruptured ovarian cyst can cause severe pain and should be considered in the differential diagnosis of a female patient.