What does the 95% confidence interval (CI) tell us about the odds ratio (OR) and statistical significance?
OR = 1.0 → No association
CI does not include 1.0 → Statistically significant (P < 0.05)
CI includes 1.0 → Not statistically significant
A 48-year-old African American male develops acute hemolytic anemia after taking trimethoprim-sulfamethoxazole. Labs show ↓Hb, ↑LDH, ↑bilirubin, and peripheral smear shows spherocytes. What is the most likely underlying etiology?
G6PD deficiency — a genetic inability to modulate oxidative stress. RBCs are prone to oxidative damage from certain drugs, infections, or fava beans.
A 65-year-old man taking an OTC antihistamine for motion sickness develops urinary retention and difficulty reading small print. What receptor besides H1 is likely being blocked?
Muscarinic (M1/M3) receptors — first-generation H1 antihistamines (diphenhydramine, dimenhydrinate, meclizine) have anticholinergic activity, causing urinary retention, blurred vision, dry mouth, and constipation.
Name some first-generation H1 antihistamines commonly used for motion sickness.
Diphenhydramine
Dimenhydrinate
Meclizine
A 45-year-old male IV drug user presents with digital necrosis, purpura on the lower extremities, arthralgias, and peripheral neuropathy. What is the most likely underlying cause?
Hepatitis C virus (HCV)–associated mixed cryoglobulinemia
Chronic HCV infection → formation of immune complexes (cryoglobulins) → small-to-medium vessel vasculitis.
What is the classic triad of symptoms in mixed cryoglobulinemia?
M → Maculopapular or palpable purpura (lower extremities)
E → Extravasation arthralgias
L → Loss of sensation / peripheral neuropathy necrosis
Weakness is often included as a fourth feature.
Which vasculitis is most closely associated with hepatitis B, and how is it different from HCV-associated cryoglobulinemia?
Polyarteritis nodosa (PAN) → medium-vessel vasculitis, often associated with hepatitis B
Unlike HCV cryoglobulinemia, PAN rarely causes digital necrosis or palpable purpura
A 24-year-old HIV-positive man with a CD4 count of 65 cells/mm³ presents with 30–40 watery stools per day for 4 weeks. Stool shows acid-fast oocysts. What is the most likely pathogen?
Cryptosporidium parvum — causes profuse, chronic watery diarrhea in patients with CD4 <100.
A 55-year-old woman has hypercalcemia, bone pain, proximal muscle weakness, and kidney stones. What hormone is most likely elevated?
Parathyroid hormone (PTH) → primary hyperparathyroidism (most commonly due to a parathyroid adenoma).
What is the main effect of PTH on the kidneys?
Inhibits sodium/phosphate cotransporter in proximal tubule → increases phosphate excretion
Increases calcium reabsorption in distal tubule
Stimulates 1α-hydroxylase → more calcitriol → increased intestinal calcium absorption
What lab pattern is characteristic of primary hyperparathyroidism?
↑ Serum calcium
↓ Serum phosphate
Elevated PTH
Sometimes elevated alkaline phosphatase if bone involvement
What is the function of the sodium/phosphate (Na⁺/Pi) cotransporter in the proximal tubule, and how is it regulated?
Function: reabsorbs phosphate (Pi) from the filtrate into the blood using the sodium gradient.
Regulation:
Inhibited by PTH → increases phosphate excretion (phosphaturia)
Inhibited by FGF23 → increases phosphate excretion
Upregulated by low dietary phosphate → more phosphate reabsorbed
Clinical relevance:
PTH-induced inhibition in primary hyperparathyroidism → low serum phosphate, phosphate wasting in urine
