My guess would be lipids, especially PUFAs (poly-unsaturated fatty acids). They are more easy to oxidise compared to nucleic acids and proteins.

Regards, Christian 

I think, lipids.. and you can more ways and methods for measuring ROS and MDA.. for example CLmethod

I believe that lipids (unsaturates) are the most susceptible biomolecules to oxidation. Protein sulfhydryl groups are also easily changed by redox imbalance. There are transcriptional factors, such as nfr2, that are redox sensitive to sulfhydryl alterations.

Dear lipids are more susceptible than others.

I concur with the answers provided for the question.

I believe it is the lipid Hence access oxidative stress,any of MDA etc

By far lipids are the most susceptible - much more so than proteins.

Even though lipids are most susceptible, they are also very unstable. Suggestion is to include one more biomolecule damage (protein or DNA) beside LPO.

This looks very dependent on whether or not anti-oxidant systems are available. Lipid peroxidation is a major problem but vitamin E and other fat-soluble antioxidants are protecting against that mean the lipids may not be the primary problem in normal status with anti-oxidants. DNA is susceptible especially in mitochondria that presumably lead to deletion during aging. Most devastating oxidative stress is produced by immune systems to kill pathogens that I would think it most susceptible at the expense of defending our body. These include NADPH oxidase and leading pathways that could result in a wide variety of pathophysiologic outcomes. The deleterious outcomes are seen especially in some health conditions. Some references of many are:

Article NADPH oxidase, oxidative stress and fibrosis in Systemic Sclerosis

Article The role of oxidative stress and NADPH oxidase in the pathog...

Article Oxidative Stress, Neuroinflammation, and NADPH Oxidase: Impl...

Sorry, I missed the mosquito that I do not know much about. Above is my thought in general.