What are the differential diagnoses for a 25-year-old male (M) with acute left-sided abdominal pain, vomiting bright yellow-green bile, recent history of cough and rhinorrhea, and taking zinc with food?

Differential Diagnoses for Acute Left-Sided Abdominal Pain with Bilious Vomiting

The most likely diagnosis in this 25-year-old male with acute left-sided abdominal pain and bilious vomiting is acute gastroenteritis, possibly zinc-induced, though acute left colonic diverticulitis and acute appendicitis must be excluded given the acute presentation and vomiting. 1

Primary Differential Diagnoses

Zinc-Induced Gastritis/Gastroenteritis

• Temporal relationship is highly suggestive: symptoms began 35 minutes after zinc ingestion with food, which is consistent with direct gastric irritation 1
• Bright yellow-green bilious vomiting occurring in three back-to-back episodes indicates gastric irritation with duodenal reflux 1
• Zinc supplementation commonly causes nausea, vomiting, and abdominal pain when taken on an empty stomach or in excessive doses 1
• The diaphoresis during vomiting episodes suggests vagal stimulation from gastric distension or irritation 2

Acute Gastroenteritis (Viral)

• The concurrent upper respiratory symptoms (weeks-long cough worsening yesterday, new rhinorrhea, nasal burning) suggest a viral prodrome that could include gastrointestinal manifestations 3
• Viral gastroenteritis commonly presents with nausea, vomiting, and abdominal pain, with diarrhea potentially developing later 3
• COVID-19 specifically can present with abdominal pain (6.6%), nausea (15.3%), and vomiting (8.3%) even in mild cases 3
• The absence of fever does not exclude viral gastroenteritis, as many cases present without fever 3

Acute Left Colonic Diverticulitis (ALCD)

• This diagnosis must be considered despite the patient's young age: up to one-fifth of ALCD patients are under 50 years 3
• Classic presentation includes acute left lower quadrant pain or tenderness 3
• However, the presence of vomiting argues against uncomplicated ALCD: absence of vomiting is one of three criteria in the Laméris clinical decision rule for ALCD diagnosis (along with direct left lower quadrant tenderness and CRP >50 mg/L) 3
• When vomiting is present with left-sided abdominal pain, it predicts progression to complicated disease and warrants imaging 3
• Clinical diagnosis of ALCD lacks accuracy (positive predictive value only 0.65 on clinical grounds alone), requiring CT imaging for confirmation 3

Acute Appendicitis

• Must be considered in any young patient with acute abdominal pain and vomiting 1, 2
• While classically right lower quadrant, appendicitis can present with diffuse or left-sided pain, particularly early in the course 2
• The combination of vomiting and abdominal pain has high sensitivity for appendicitis 1
• Normal bowel movements do not exclude appendicitis 2

Secondary Differential Diagnoses

Small Bowel Obstruction

• Bilious vomiting (bright yellow-green) is characteristic of proximal small bowel obstruction 1
• However, the absence of abdominal distension, constipation, and the acute onset without prior abdominal surgery (which has 85% sensitivity for adhesive obstruction) make this less likely 1
• Normal bowel movements further argue against obstruction 1

Acute Hemorrhagic Colitis (COVID-19 Associated)

• Recent data from the Omicron variant phase shows acute hemorrhagic colitis characteristically affects the left-sided colon in COVID-19 patients with gastrointestinal bleeding 4
• However, this patient denies gastrointestinal bleeding, making this diagnosis unlikely 4

Inflammatory Bowel Disease (IBD) - Initial Presentation

• Can present with left-sided abdominal pain, vomiting, and systemic symptoms 1
• The concurrent respiratory symptoms and acute onset make this less likely as a first presentation 1

Critical Next Steps in Evaluation

Immediate Assessment Required

• Vital signs with orthostatic measurements to assess volume status from vomiting 1
• Abdominal examination specifically for:
  • Localization of tenderness (diffuse vs. left lower quadrant specific) 3
  • Peritoneal signs (rebound, guarding) which would indicate complicated disease 3
  • Abdominal distension 1
  • Bowel sounds 1

Laboratory Studies Indicated

• Complete blood count (CBC) to evaluate for leukocytosis suggesting infection or inflammation 1
• Comprehensive metabolic panel including electrolytes (given vomiting), renal function, glucose, and liver enzymes 1
• C-reactive protein (CRP) if ALCD is suspected: CRP >50 mg/L supports diverticulitis diagnosis, while CRP >170 mg/L predicts severe/complicated disease 3
• Consider lactate if peritoneal signs present to evaluate for bowel ischemia 1

Imaging Strategy

• CT abdomen/pelvis with IV contrast is indicated if:
  • Localized left lower quadrant tenderness persists 3
  • Any peritoneal signs are present 3, 1
  • Vomiting continues despite conservative management 3
  • Diagnostic uncertainty remains after initial evaluation 3
• CT has 99% sensitivity for appendicitis and >95% positive predictive value for ALCD 3
• Ultrasound can be considered first if CT unavailable, with 90% sensitivity and specificity for ALCD 3

Red Flags Requiring Immediate Intervention

• Fever ≥38°C developing would indicate bacterial infection requiring immediate reassessment 5
• Peritoneal signs (rebound tenderness, involuntary guarding) mandate surgical consultation 1
• Hemodynamic instability or signs of severe dehydration 1
• Worsening or severe localized pain despite conservative management 3

Common Pitfalls to Avoid

• Do not dismiss ALCD based on age alone: while more common in older patients, 20% of cases occur in patients under 50 years 3
• Do not assume zinc toxicity is benign: while likely self-limited, severe gastric irritation can mimic surgical conditions 1
• Do not rely on clinical examination alone for ALCD: clinical diagnosis has only 65% positive predictive value without imaging 3
• Do not ignore the vomiting: its presence with left-sided pain predicts complicated diverticulitis and progression to surgery 3
• Do not attribute all symptoms to concurrent URI: COVID-19 and other viral illnesses can present primarily with GI symptoms 3