Acute Appendicitis: The patient's symptoms of severe abdominal pain, initially diffuse and then localized to the right lower quadrant, fever, tachycardia, and physical examination findings (severe abdominal tenderness, guarding, rebound, positive McBurney and Psoas signs) are classic for acute appendicitis. The condition is common in adolescents and the presentation matches the typical clinical picture.

Mesenteric Lymphadenitis: This condition can mimic appendicitis, presenting with abdominal pain, fever, and similar physical examination findings. It is often related to viral infections and can be difficult to distinguish from appendicitis without imaging.
Intestinal Obstruction: Although less likely given the acute onset and specific localization of pain, intestinal obstruction could present with severe abdominal pain, vomiting, and altered bowel habits. The presence of fever and tachycardia could suggest a complicated obstruction.
Gastroenteritis: While typically presenting with diarrhea and vomiting, severe gastroenteritis can cause significant abdominal pain and fever. However, the localized tenderness and specific signs like McBurney and Psoas suggest a more localized process.

Ectopic Pregnancy (in female patients, not applicable here): Not relevant in this case due to the patient's gender, but always a critical consideration in females of childbearing age with abdominal pain.
Perforated Viscus: Any condition leading to perforation of the gastrointestinal tract (e.g., perforated ulcer, typhoid fever) can present with severe abdominal pain, fever, and peritoneal signs. This is a surgical emergency.
Intra-abdominal Shunt or Abscess: Complications of conditions like inflammatory bowel disease or previous surgeries can lead to abscess formation or shunt malfunction, presenting with severe abdominal pain and systemic signs of infection.

Yersiniosis: A bacterial infection that can cause abdominal pain, fever, and sometimes mimic appendicitis, especially if the infection involves the terminal ileum.
Ovarian Torsion (not applicable here): Again, not relevant due to the patient's gender, but an important consideration in adolescent females with acute abdominal pain.
Meckel's Diverticulitis: A congenital anomaly of the small intestine that can become inflamed and present similarly to appendicitis, though it is less common.
Primary Peritonitis: Also known as spontaneous bacterial peritonitis, it is more common in patients with cirrhosis or immunocompromised states but can occur in otherwise healthy individuals, presenting with diffuse abdominal pain and signs of peritonitis.