Which organism is commonly detected in stool using acid-fast staining due to thick-walled oocysts?

• A. Cryptosporidium parvum
• B. Giardia lamblia
• C. Entamoeba histolytica
• D. Ascaris lumbricoides

Answer: (A)

Acid-fast staining targets organisms with thick-walled oocysts that resist decolorization, a feature of certain coccidian parasites. In stool samples, Cryptosporidium parvum forms small, thick-walled oocysts that reliably retain the stain with the modified acid-fast (Ziehl-Neelsen) method, making them easy to spot as bright red/pink bodies against a blue background.

This method isn’t used for Giardia lamblia, Entamoeba histolytica, or Ascaris lumbricoides, whose diagnostic forms aren’t acid-fast. Giardia cysts and trophozoites, Entamoeba histolytica cysts, and Ascaris eggs are detected by other stains and techniques. Therefore, the organism commonly detected in stool using acid-fast staining due to thick-walled oocysts is Cryptosporidium parvum.