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Myasthenia gravis is an autoimmune neuromuscular disorder characterized by muscle weakness and fatigue. Pyridostigmine, commonly sold under the brand name Mestinon, is a medication used to treat myasthenia gravis. This drug is an acetylcholinesterase inhibitor, which means it increases the levels of acetylcholine in the synaptic cleft, improving the communication between nerve cells and muscle fibers.

Let's examine each of the given options one-by-one: A. Muscle cramps - Pyridostigmine could cause muscle cramps in some cases, as it leads to an increased level of acetylcholine, which in turn can cause increased muscle contractions. B. Dry mouth - Dry mouth is not among the commonly reported side effects of pyridostigmine, as the increase in acetylcholine can act on muscarinic receptors in the salivary glands, increasing secretion. C. Decreased lacrimation - Decreased tearing is also not a common side effect of pyridostigmine, as the drug can also stimulate muscarinic receptors in the lacrimal glands, which increase the production of tears. D. Hyperactivity episodes - Hyperactivity episodes are not typically reported as a side effect of pyridostigmine, as the drug acts on neuromuscular communication rather than the central nervous system.

Based on the above analysis, option A (Muscle cramps) is the most plausible side effect of pyridostigmine that the client would complain about. So, the correct answer is: A. Muscle cramps