Okay, let's tackle this question. The patient is a 5-month-old with colicky pain, bilious vomiting, and red currant jelly stools. The physical exam shows a sausage-shaped mass in the right lumbar region. These are classic signs of intussusception. Intussusception is when a part of the intestine telescopes into another part, leading to obstruction and possible strangulation. The red currant jelly stool is a key indicator here.

The question asks for a diagnostic and therapeutic modality. The options aren't given, but the correct answer is likely air or barium enema. Both can be used to both diagnose and treat intussusception by reducing the telescoped segment.

Wait, the options might include other imaging techniques. Let me think. Ultrasound is a diagnostic tool but not therapeutic. Laparotomy would be a treatment but not diagnostic. So the correct answer should be air enema. Let me confirm: air enema is used for both diagnosis and treatment in intussusception, especially in infants. It's less invasive than surgery and can resolve the issue if done early.

Now, the incorrect options: If an option is CT scan, that's diagnostic only. If there's a surgical option, it's treatment but not diagnostic. Barium enema might also be an option but air enema is preferred. So the correct answer is air enema. The clinical pearl here is that red currant jelly stools in an infant with a sausage-shaped mass points to intussusception, and air enema is the go-to for both diagnosis and treatment.

**Core Concept** Intussusception in infants is characterized by colicky abdominal pain, bilious vomiting, red currant jelly stools, and a palpable sausage-shaped mass. The primary diagnostic and therapeutic modality involves imaging-guided reduction.

**Why the Correct Answer is Right** Air enema is both diagnostic and therapeutic for intussusception. It visualizes the telescoped bowel and applies pressure to reduce the obstruction, avoiding surgery in most cases. It is safe in hemodynamically stable infants with no signs of peritonitis or bowel perforation.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound confirms the "target sign" but does not treat the condition.
**Option B:** Laparotomy is a definitive treatment but requires surgical intervention rather than non-invasive diagnosis.
**Option C:** CT scan is not used in infants due to radiation risk and lacks therapeutic utility.

**Clinical Pearl / High-Yield Fact** Remember the "rule of threes": intussusception occurs in 3–12 months, has 3 phases (obstruction, ischemia, perforation), and presents with 3 classic symptoms (vomiting, pain, currant jelly stools). Air enema is contraindicated if there are signs of peritonitis or shock.

**Correct Answer: D. Air enema**

✓ Correct Answer: B. Air enema