This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Forensic Medicine
For diatoms test, the best site for taking a sample is - 
Select and explain. 
A. Lungs, B. Bone marrow in Ulna, C. Bone marrow in femur, D. Muscle
C: Bone marrow in femur
Answer: C: Bone marrow in femur 
Explanation: Diatoms can be demonstrated in all the given options.

"The current technique for isolation of diatoms involve acid digestion of tissue-commonly bone marrow, lung, blood or kidneys with subsequent centrifugation and washing." - Parikh
"Bone marrow of long bone (e.g. femur) is examined for diatoms. Kidney, lung, liver or brain is also washed and 1 x1 cm, pieces from the deeper tissue." - Reddy

But, the examiner is specifically asking for ideal tissue, which is bone marrow of long bones:

­"The bone marrow is highly suitable and reliable. The bone marrow of long bones, such as femur, tibia, and humerus or sternum is examined for diatoms" - Reddy