What is the likely diagnosis for a patient presenting with bloody diarrhea, a positive family history of the same condition, and a positive guaiac (fecal occult blood) test?

Likely Diagnosis: Familial Adenomatous Polyposis (FAP)

The most likely diagnosis is C) Familial polyposis (Familial Adenomatous Polyposis), given the combination of bloody diarrhea, positive family history of the same condition, and positive guaiac test indicating gastrointestinal bleeding from multiple adenomatous polyps.

Clinical Reasoning

Why FAP is the Most Likely Diagnosis

• FAP is characterized by the development of hundreds to thousands of adenomatous polyps in the colon and rectum, typically manifesting during the second decade of life 1
• Rectal bleeding is a common presenting symptom when adenomas become large and numerous, often accompanied by diarrhea, abdominal pain, and anemia 1, 2
• FAP is inherited in an autosomal dominant manner with approximately 70% of patients having a family history of colorectal polyps and cancer 1
• The positive guaiac test confirms gastrointestinal bleeding, which aligns with the bleeding polyps characteristic of FAP 1, 3

Why Other Options Are Less Likely

Ulcerative colitis (Option A):

• While ulcerative colitis can present with bloody diarrhea and has familial clustering, the question specifically states that relatives have "the same condition" 4
• The familial pattern in FAP is much more predictable and follows autosomal dominant inheritance, whereas ulcerative colitis has more complex genetic susceptibility 1

Diverticulosis (Option B):

• Diverticulosis typically presents in older adults and is not a hereditary condition with clear familial transmission 4
• Bleeding from diverticulosis is usually acute and painless, not associated with chronic diarrhea 1

Peutz-Jeghers syndrome (Option D):

• While this is a hereditary polyposis syndrome, it is characterized by hamartomatous polyps (not adenomatous) and distinctive mucocutaneous pigmentation 4
• The polyps in Peutz-Jeghers are less likely to cause the degree of bleeding and diarrhea seen with the numerous adenomas of FAP 4, 1

Key Clinical Features Supporting FAP Diagnosis

• Nonspecific gastrointestinal symptoms including diarrhea, rectal bleeding, abdominal pain, and potential anemia are hallmark presentations 1, 2
• Patients are often asymptomatic for years until adenomas become large and numerous 1
• The positive family history is critical, as FAP families require genetic counseling and testing of all first-degree relatives 4, 1

Important Next Steps

• Immediate colonoscopy is required for definitive diagnosis and visualization of the characteristic multiple adenomatous polyps 4, 5
• Genetic testing for APC gene mutations should be performed to confirm the diagnosis and enable family screening 4, 1, 6
• Referral to a Regional Genetics Centre for formal counseling is mandatory for both the patient and family members 4

Common Pitfalls to Avoid

• Do not delay colonoscopy after a positive guaiac test—proceed directly to diagnostic evaluation within 60 days 5
• Do not assume all familial gastrointestinal bleeding is inflammatory bowel disease; consider hereditary polyposis syndromes when multiple family members are affected 4
• Remember that FAP carries a 100% lifetime risk of colorectal cancer without prophylactic colectomy, making early diagnosis critical 1, 6